SERVER-31608 future-proof FCV enum

26 files changed, 138 insertions, 100 deletions

diff --git a/src/mongo/db/auth/authorization_session_test.cpp b/src/mongo/db/auth/authorization_session_test.cpp
index 9fb7effb64c..9a77a25de18 100644
--- a/src/mongo/db/auth/authorization_session_test.cpp
+++ b/src/mongo/db/auth/authorization_session_test.cpp
@@ -94,7 +94,7 @@ public:
 
     void setUp() {
         serverGlobalParams.featureCompatibility.setVersion(
-            ServerGlobalParams::FeatureCompatibility::Version::k36);
+            ServerGlobalParams::FeatureCompatibility::Version::kFullyUpgradedTo36);
         session = transportLayer.createSession();
         client = serviceContext.makeClient("testClient", session);
         RestrictionEnvironment::set(
diff --git a/src/mongo/db/auth/role_graph_test.cpp b/src/mongo/db/auth/role_graph_test.cpp
index dcbe03002ca..5a8faa3c1b3 100644
--- a/src/mongo/db/auth/role_graph_test.cpp
+++ b/src/mongo/db/auth/role_graph_test.cpp
@@ -775,7 +775,7 @@ TEST(RoleGraphTest, AddRoleFromDocument) {
     };
 
     serverGlobalParams.featureCompatibility.setVersion(
-        ServerGlobalParams::FeatureCompatibility::Version::k36);
+        ServerGlobalParams::FeatureCompatibility::Version::kFullyUpgradedTo36);
 
     RoleGraph graph;
 
diff --git a/src/mongo/db/auth/user_document_parser_test.cpp b/src/mongo/db/auth/user_document_parser_test.cpp
index 2eed6173e0a..ab410823dea 100644
--- a/src/mongo/db/auth/user_document_parser_test.cpp
+++ b/src/mongo/db/auth/user_document_parser_test.cpp
@@ -232,7 +232,7 @@ public:
 
     void setUp() {
         serverGlobalParams.featureCompatibility.setVersion(
-            ServerGlobalParams::FeatureCompatibility::Version::k36);
+            ServerGlobalParams::FeatureCompatibility::Version::kFullyUpgradedTo36);
         user.reset(new User(UserName("spencer", "test")));
         adminUser.reset(new User(UserName("admin", "admin")));
     }
@@ -646,7 +646,7 @@ TEST_F(V2UserDocumentParsing, V2AuthenticationRestrictionsExtraction) {
 
 TEST_F(V2UserDocumentParsing, V2AuthenticationRestrictionsExtractionAndRetreival) {
     serverGlobalParams.featureCompatibility.setVersion(
-        ServerGlobalParams::FeatureCompatibility::Version::k36);
+        ServerGlobalParams::FeatureCompatibility::Version::kFullyUpgradedTo36);
     enableIPv6(true);
     ASSERT_OK(v2parser.initializeAuthenticationRestrictionsFromUserDocument(
         BSON("user"
diff --git a/src/mongo/db/catalog/index_spec_validate_test.cpp b/src/mongo/db/catalog/index_spec_validate_test.cpp
index be5c1135818..764990696c6 100644
--- a/src/mongo/db/catalog/index_spec_validate_test.cpp
+++ b/src/mongo/db/catalog/index_spec_validate_test.cpp
@@ -66,7 +66,8 @@ BSONObj sorted(const BSONObj& obj) {
 
 TEST(IndexSpecValidateTest, ReturnsAnErrorIfKeyPatternIsNotAnObject) {
     ServerGlobalParams::FeatureCompatibility featureCompatibility;
-    featureCompatibility.setVersion(ServerGlobalParams::FeatureCompatibility::Version::k36);
+    featureCompatibility.setVersion(
+        ServerGlobalParams::FeatureCompatibility::Version::kFullyUpgradedTo36);
 
     ASSERT_EQ(ErrorCodes::TypeMismatch,
               validateIndexSpec(kDefaultOpCtx,
@@ -92,7 +93,8 @@ TEST(IndexSpecValidateTest, ReturnsAnErrorIfKeyPatternIsNotAnObject) {
 
 TEST(IndexSpecValidateTest, ReturnsAnErrorIfFieldRepeatedInKeyPattern) {
     ServerGlobalParams::FeatureCompatibility featureCompatibility;
-    featureCompatibility.setVersion(ServerGlobalParams::FeatureCompatibility::Version::k36);
+    featureCompatibility.setVersion(
+        ServerGlobalParams::FeatureCompatibility::Version::kFullyUpgradedTo36);
 
     ASSERT_EQ(ErrorCodes::BadValue,
               validateIndexSpec(kDefaultOpCtx,
@@ -112,7 +114,8 @@ TEST(IndexSpecValidateTest, ReturnsAnErrorIfFieldRepeatedInKeyPattern) {
 
 TEST(IndexSpecValidateTest, ReturnsAnErrorIfKeyPatternIsNotPresent) {
     ServerGlobalParams::FeatureCompatibility featureCompatibility;
-    featureCompatibility.setVersion(ServerGlobalParams::FeatureCompatibility::Version::k36);
+    featureCompatibility.setVersion(
+        ServerGlobalParams::FeatureCompatibility::Version::kFullyUpgradedTo36);
 
     ASSERT_EQ(ErrorCodes::FailedToParse,
               validateIndexSpec(kDefaultOpCtx,
@@ -124,7 +127,8 @@ TEST(IndexSpecValidateTest, ReturnsAnErrorIfKeyPatternIsNotPresent) {
 
 TEST(IndexSpecValidateTest, ReturnsAnErrorIfNameIsNotAString) {
     ServerGlobalParams::FeatureCompatibility featureCompatibility;
-    featureCompatibility.setVersion(ServerGlobalParams::FeatureCompatibility::Version::k36);
+    featureCompatibility.setVersion(
+        ServerGlobalParams::FeatureCompatibility::Version::kFullyUpgradedTo36);
 
     ASSERT_EQ(ErrorCodes::TypeMismatch,
               validateIndexSpec(kDefaultOpCtx,
@@ -135,7 +139,8 @@ TEST(IndexSpecValidateTest, ReturnsAnErrorIfNameIsNotAString) {
 
 TEST(IndexSpecValidateTest, ReturnsAnErrorIfNameIsNotPresent) {
     ServerGlobalParams::FeatureCompatibility featureCompatibility;
-    featureCompatibility.setVersion(ServerGlobalParams::FeatureCompatibility::Version::k36);
+    featureCompatibility.setVersion(
+        ServerGlobalParams::FeatureCompatibility::Version::kFullyUpgradedTo36);
 
     ASSERT_EQ(ErrorCodes::FailedToParse,
               validateIndexSpec(kDefaultOpCtx,
@@ -146,7 +151,8 @@ TEST(IndexSpecValidateTest, ReturnsAnErrorIfNameIsNotPresent) {
 
 TEST(IndexSpecValidateTest, ReturnsAnErrorIfNamespaceIsNotAString) {
     ServerGlobalParams::FeatureCompatibility featureCompatibility;
-    featureCompatibility.setVersion(ServerGlobalParams::FeatureCompatibility::Version::k36);
+    featureCompatibility.setVersion(
+        ServerGlobalParams::FeatureCompatibility::Version::kFullyUpgradedTo36);
 
     ASSERT_EQ(ErrorCodes::TypeMismatch,
               validateIndexSpec(kDefaultOpCtx,
@@ -168,7 +174,8 @@ TEST(IndexSpecValidateTest, ReturnsAnErrorIfNamespaceIsNotAString) {
 
 TEST(IndexSpecValidateTest, ReturnsAnErrorIfNamespaceIsEmptyString) {
     ServerGlobalParams::FeatureCompatibility featureCompatibility;
-    featureCompatibility.setVersion(ServerGlobalParams::FeatureCompatibility::Version::k36);
+    featureCompatibility.setVersion(
+        ServerGlobalParams::FeatureCompatibility::Version::kFullyUpgradedTo36);
 
     ASSERT_EQ(ErrorCodes::BadValue,
               validateIndexSpec(kDefaultOpCtx,
@@ -182,7 +189,8 @@ TEST(IndexSpecValidateTest, ReturnsAnErrorIfNamespaceIsEmptyString) {
 
 TEST(IndexSpecValidateTest, ReturnsAnErrorIfNamespaceDoesNotMatch) {
     ServerGlobalParams::FeatureCompatibility featureCompatibility;
-    featureCompatibility.setVersion(ServerGlobalParams::FeatureCompatibility::Version::k36);
+    featureCompatibility.setVersion(
+        ServerGlobalParams::FeatureCompatibility::Version::kFullyUpgradedTo36);
 
     ASSERT_EQ(ErrorCodes::BadValue,
               validateIndexSpec(kDefaultOpCtx,
@@ -207,7 +215,8 @@ TEST(IndexSpecValidateTest, ReturnsAnErrorIfNamespaceDoesNotMatch) {
 
 TEST(IndexSpecValidateTest, ReturnsIndexSpecWithNamespaceFilledInIfItIsNotPresent) {
     ServerGlobalParams::FeatureCompatibility featureCompatibility;
-    featureCompatibility.setVersion(ServerGlobalParams::FeatureCompatibility::Version::k36);
+    featureCompatibility.setVersion(
+        ServerGlobalParams::FeatureCompatibility::Version::kFullyUpgradedTo36);
 
     auto result = validateIndexSpec(kDefaultOpCtx,
                                     BSON("key" << BSON("field" << 1) << "name"
@@ -234,7 +243,8 @@ TEST(IndexSpecValidateTest, ReturnsIndexSpecWithNamespaceFilledInIfItIsNotPresen
 
 TEST(IndexSpecValidateTest, ReturnsIndexSpecUnchangedIfNamespaceAndVersionArePresent) {
     ServerGlobalParams::FeatureCompatibility featureCompatibility;
-    featureCompatibility.setVersion(ServerGlobalParams::FeatureCompatibility::Version::k36);
+    featureCompatibility.setVersion(
+        ServerGlobalParams::FeatureCompatibility::Version::kFullyUpgradedTo36);
 
     auto result = validateIndexSpec(kDefaultOpCtx,
                                     BSON("key" << BSON("field" << 1) << "name"
@@ -259,7 +269,8 @@ TEST(IndexSpecValidateTest, ReturnsIndexSpecUnchangedIfNamespaceAndVersionArePre
 
 TEST(IndexSpecValidateTest, ReturnsAnErrorIfVersionIsNotANumber) {
     ServerGlobalParams::FeatureCompatibility featureCompatibility;
-    featureCompatibility.setVersion(ServerGlobalParams::FeatureCompatibility::Version::k36);
+    featureCompatibility.setVersion(
+        ServerGlobalParams::FeatureCompatibility::Version::kFullyUpgradedTo36);
 
     ASSERT_EQ(ErrorCodes::TypeMismatch,
               validateIndexSpec(kDefaultOpCtx,
@@ -281,7 +292,8 @@ TEST(IndexSpecValidateTest, ReturnsAnErrorIfVersionIsNotANumber) {
 
 TEST(IndexSpecValidateTest, ReturnsAnErrorIfVersionIsNotRepresentableAsInt) {
     ServerGlobalParams::FeatureCompatibility featureCompatibility;
-    featureCompatibility.setVersion(ServerGlobalParams::FeatureCompatibility::Version::k36);
+    featureCompatibility.setVersion(
+        ServerGlobalParams::FeatureCompatibility::Version::kFullyUpgradedTo36);
 
     ASSERT_EQ(ErrorCodes::BadValue,
               validateIndexSpec(kDefaultOpCtx,
@@ -319,7 +331,8 @@ TEST(IndexSpecValidateTest, ReturnsAnErrorIfVersionIsNotRepresentableAsInt) {
 
 TEST(IndexSpecValidateTest, ReturnsAnErrorIfVersionIsV0) {
     ServerGlobalParams::FeatureCompatibility featureCompatibility;
-    featureCompatibility.setVersion(ServerGlobalParams::FeatureCompatibility::Version::k36);
+    featureCompatibility.setVersion(
+        ServerGlobalParams::FeatureCompatibility::Version::kFullyUpgradedTo36);
 
     ASSERT_EQ(ErrorCodes::CannotCreateIndex,
               validateIndexSpec(kDefaultOpCtx,
@@ -333,7 +346,8 @@ TEST(IndexSpecValidateTest, ReturnsAnErrorIfVersionIsV0) {
 
 TEST(IndexSpecValidateTest, ReturnsAnErrorIfVersionIsUnsupported) {
     ServerGlobalParams::FeatureCompatibility featureCompatibility;
-    featureCompatibility.setVersion(ServerGlobalParams::FeatureCompatibility::Version::k36);
+    featureCompatibility.setVersion(
+        ServerGlobalParams::FeatureCompatibility::Version::kFullyUpgradedTo36);
 
     ASSERT_EQ(ErrorCodes::CannotCreateIndex,
               validateIndexSpec(kDefaultOpCtx,
@@ -359,7 +373,8 @@ TEST(IndexSpecValidateTest, ReturnsAnErrorIfVersionIsUnsupported) {
 
 TEST(IndexSpecValidateTest, AcceptsIndexVersionsThatAreAllowedForCreation) {
     ServerGlobalParams::FeatureCompatibility featureCompatibility;
-    featureCompatibility.setVersion(ServerGlobalParams::FeatureCompatibility::Version::k36);
+    featureCompatibility.setVersion(
+        ServerGlobalParams::FeatureCompatibility::Version::kFullyUpgradedTo36);
 
     auto result = validateIndexSpec(kDefaultOpCtx,
                                     BSON("key" << BSON("field" << 1) << "name"
@@ -400,7 +415,8 @@ TEST(IndexSpecValidateTest, AcceptsIndexVersionsThatAreAllowedForCreation) {
 
 TEST(IndexSpecValidateTest, DefaultIndexVersionIsV2) {
     ServerGlobalParams::FeatureCompatibility featureCompatibility;
-    featureCompatibility.setVersion(ServerGlobalParams::FeatureCompatibility::Version::k36);
+    featureCompatibility.setVersion(
+        ServerGlobalParams::FeatureCompatibility::Version::kFullyUpgradedTo36);
 
     auto result = validateIndexSpec(kDefaultOpCtx,
                                     BSON("key" << BSON("field" << 1) << "name"
@@ -427,7 +443,8 @@ TEST(IndexSpecValidateTest, DefaultIndexVersionIsV2) {
 
 TEST(IndexSpecValidateTest, AcceptsIndexVersionV1) {
     ServerGlobalParams::FeatureCompatibility featureCompatibility;
-    featureCompatibility.setVersion(ServerGlobalParams::FeatureCompatibility::Version::k36);
+    featureCompatibility.setVersion(
+        ServerGlobalParams::FeatureCompatibility::Version::kFullyUpgradedTo36);
 
     auto result = validateIndexSpec(kDefaultOpCtx,
                                     BSON("key" << BSON("field" << 1) << "name"
@@ -450,7 +467,8 @@ TEST(IndexSpecValidateTest, AcceptsIndexVersionV1) {
 
 TEST(IndexSpecValidateTest, ReturnsAnErrorIfCollationIsNotAnObject) {
     ServerGlobalParams::FeatureCompatibility featureCompatibility;
-    featureCompatibility.setVersion(ServerGlobalParams::FeatureCompatibility::Version::k36);
+    featureCompatibility.setVersion(
+        ServerGlobalParams::FeatureCompatibility::Version::kFullyUpgradedTo36);
 
     ASSERT_EQ(ErrorCodes::TypeMismatch,
               validateIndexSpec(kDefaultOpCtx,
@@ -480,7 +498,8 @@ TEST(IndexSpecValidateTest, ReturnsAnErrorIfCollationIsNotAnObject) {
 
 TEST(IndexSpecValidateTest, ReturnsAnErrorIfCollationIsEmpty) {
     ServerGlobalParams::FeatureCompatibility featureCompatibility;
-    featureCompatibility.setVersion(ServerGlobalParams::FeatureCompatibility::Version::k36);
+    featureCompatibility.setVersion(
+        ServerGlobalParams::FeatureCompatibility::Version::kFullyUpgradedTo36);
 
     ASSERT_EQ(ErrorCodes::BadValue,
               validateIndexSpec(kDefaultOpCtx,
@@ -494,7 +513,8 @@ TEST(IndexSpecValidateTest, ReturnsAnErrorIfCollationIsEmpty) {
 
 TEST(IndexSpecValidateTest, ReturnsAnErrorIfCollationIsPresentAndVersionIsLessThanV2) {
     ServerGlobalParams::FeatureCompatibility featureCompatibility;
-    featureCompatibility.setVersion(ServerGlobalParams::FeatureCompatibility::Version::k36);
+    featureCompatibility.setVersion(
+        ServerGlobalParams::FeatureCompatibility::Version::kFullyUpgradedTo36);
 
     ASSERT_EQ(ErrorCodes::CannotCreateIndex,
               validateIndexSpec(kDefaultOpCtx,
@@ -511,7 +531,8 @@ TEST(IndexSpecValidateTest, ReturnsAnErrorIfCollationIsPresentAndVersionIsLessTh
 
 TEST(IndexSpecValidateTest, AcceptsAnyNonEmptyObjectValueForCollation) {
     ServerGlobalParams::FeatureCompatibility featureCompatibility;
-    featureCompatibility.setVersion(ServerGlobalParams::FeatureCompatibility::Version::k36);
+    featureCompatibility.setVersion(
+        ServerGlobalParams::FeatureCompatibility::Version::kFullyUpgradedTo36);
 
     auto result = validateIndexSpec(kDefaultOpCtx,
                                     BSON("key" << BSON("field" << 1) << "name"
@@ -562,7 +583,8 @@ TEST(IndexSpecValidateTest, AcceptsAnyNonEmptyObjectValueForCollation) {
 
 TEST(IndexSpecValidateTest, AcceptsIndexSpecIfCollationIsPresentAndVersionIsEqualToV2) {
     ServerGlobalParams::FeatureCompatibility featureCompatibility;
-    featureCompatibility.setVersion(ServerGlobalParams::FeatureCompatibility::Version::k36);
+    featureCompatibility.setVersion(
+        ServerGlobalParams::FeatureCompatibility::Version::kFullyUpgradedTo36);
 
     auto result = validateIndexSpec(kDefaultOpCtx,
                                     BSON("key" << BSON("field" << 1) << "name"
@@ -591,7 +613,8 @@ TEST(IndexSpecValidateTest, AcceptsIndexSpecIfCollationIsPresentAndVersionIsEqua
 
 TEST(IndexSpecValidateTest, ReturnsAnErrorIfUnknownFieldIsPresentInSpecV2) {
     ServerGlobalParams::FeatureCompatibility featureCompatibility;
-    featureCompatibility.setVersion(ServerGlobalParams::FeatureCompatibility::Version::k36);
+    featureCompatibility.setVersion(
+        ServerGlobalParams::FeatureCompatibility::Version::kFullyUpgradedTo36);
 
     auto result = validateIndexSpec(kDefaultOpCtx,
                                     BSON("key" << BSON("field" << 1) << "name"
@@ -607,7 +630,8 @@ TEST(IndexSpecValidateTest, ReturnsAnErrorIfUnknownFieldIsPresentInSpecV2) {
 
 TEST(IndexSpecValidateTest, ReturnsAnErrorIfUnknownFieldIsPresentInSpecV1) {
     ServerGlobalParams::FeatureCompatibility featureCompatibility;
-    featureCompatibility.setVersion(ServerGlobalParams::FeatureCompatibility::Version::k36);
+    featureCompatibility.setVersion(
+        ServerGlobalParams::FeatureCompatibility::Version::kFullyUpgradedTo36);
 
     auto result = validateIndexSpec(kDefaultOpCtx,
                                     BSON("key" << BSON("field" << 1) << "name"
@@ -844,7 +868,8 @@ TEST(IndexSpecCollationValidateTest, FillsInCollationFieldWithCollectionDefaultI
 
 TEST(IndexSpecPartialFilterTest, FailsIfPartialFilterIsNotAnObject) {
     ServerGlobalParams::FeatureCompatibility featureCompatibility;
-    featureCompatibility.setVersion(ServerGlobalParams::FeatureCompatibility::Version::k36);
+    featureCompatibility.setVersion(
+        ServerGlobalParams::FeatureCompatibility::Version::kFullyUpgradedTo36);
 
     auto result = validateIndexSpec(kDefaultOpCtx,
                                     BSON("key" << BSON("field" << 1) << "name"
@@ -858,7 +883,8 @@ TEST(IndexSpecPartialFilterTest, FailsIfPartialFilterIsNotAnObject) {
 
 TEST(IndexSpecPartialFilterTest, FailsIfPartialFilterContainsBannedFeature) {
     ServerGlobalParams::FeatureCompatibility featureCompatibility;
-    featureCompatibility.setVersion(ServerGlobalParams::FeatureCompatibility::Version::k36);
+    featureCompatibility.setVersion(
+        ServerGlobalParams::FeatureCompatibility::Version::kFullyUpgradedTo36);
 
     auto result = validateIndexSpec(kDefaultOpCtx,
                                     BSON("key" << BSON("field" << 1) << "name"
@@ -872,7 +898,8 @@ TEST(IndexSpecPartialFilterTest, FailsIfPartialFilterContainsBannedFeature) {
 
 TEST(IndexSpecPartialFilterTest, AcceptsValidPartialFilterExpression) {
     ServerGlobalParams::FeatureCompatibility featureCompatibility;
-    featureCompatibility.setVersion(ServerGlobalParams::FeatureCompatibility::Version::k36);
+    featureCompatibility.setVersion(
+        ServerGlobalParams::FeatureCompatibility::Version::kFullyUpgradedTo36);
 
     auto result = validateIndexSpec(kDefaultOpCtx,
                                     BSON("key" << BSON("field" << 1) << "name"
diff --git a/src/mongo/db/commands/feature_compatibility_version.cpp b/src/mongo/db/commands/feature_compatibility_version.cpp
index 0bfb943fc6b..10444bea57c 100644
--- a/src/mongo/db/commands/feature_compatibility_version.cpp
+++ b/src/mongo/db/commands/feature_compatibility_version.cpp
@@ -62,7 +62,7 @@ Lock::ResourceMutex FeatureCompatibilityVersion::fcvLock("featureCompatibilityVe
 StatusWith<ServerGlobalParams::FeatureCompatibility::Version> FeatureCompatibilityVersion::parse(
     const BSONObj& featureCompatibilityVersionDoc) {
     ServerGlobalParams::FeatureCompatibility::Version version =
-        ServerGlobalParams::FeatureCompatibility::Version::kUnset;
+        ServerGlobalParams::FeatureCompatibility::Version::kUnsetDefault34Behavior;
     std::string versionString;
     std::string targetVersionString;
 
@@ -133,7 +133,7 @@ StatusWith<ServerGlobalParams::FeatureCompatibility::Version> FeatureCompatibili
         } else if (targetVersionString == FeatureCompatibilityVersionCommandParser::kVersion34) {
             version = ServerGlobalParams::FeatureCompatibility::Version::kDowngradingTo34;
         } else {
-            version = ServerGlobalParams::FeatureCompatibility::Version::k34;
+            version = ServerGlobalParams::FeatureCompatibility::Version::kFullyDowngradedTo34;
         }
 
     } else if (versionString == FeatureCompatibilityVersionCommandParser::kVersion36) {
@@ -150,7 +150,7 @@ StatusWith<ServerGlobalParams::FeatureCompatibility::Version> FeatureCompatibili
                                         << feature_compatibility_version::kDochubLink
                                         << ".");
         } else {
-            version = ServerGlobalParams::FeatureCompatibility::Version::k36;
+            version = ServerGlobalParams::FeatureCompatibility::Version::kFullyUpgradedTo36;
         }
     } else {
         return Status(ErrorCodes::BadValue,
@@ -297,7 +297,7 @@ void uassertDuringInvalidUpgradeOp(OperationContext* opCtx,
         return;
     }
 
-    if (version != ServerGlobalParams::FeatureCompatibility::Version::k36) {
+    if (version != ServerGlobalParams::FeatureCompatibility::Version::kFullyUpgradedTo36) {
         // Only check this assertion if we're completing an upgrade to 3.6.
         return;
     }
@@ -346,7 +346,7 @@ void FeatureCompatibilityVersion::onInsertOrUpdate(OperationContext* opCtx, cons
         serverGlobalParams.featureCompatibility.setVersion(newVersion);
 
         // Close all connections from internal clients with binary versions lower than 3.6.
-        if (newVersion == ServerGlobalParams::FeatureCompatibility::Version::k36 ||
+        if (newVersion == ServerGlobalParams::FeatureCompatibility::Version::kFullyUpgradedTo36 ||
             newVersion == ServerGlobalParams::FeatureCompatibility::Version::kUpgradingTo36) {
             opCtx->getServiceContext()->getServiceEntryPoint()->endAllSessions(
                 transport::Session::kLatestVersionInternalClientKeepOpen |
@@ -371,7 +371,7 @@ void FeatureCompatibilityVersion::onDelete(OperationContext* opCtx, const BSONOb
           << FeatureCompatibilityVersionCommandParser::kVersion34;
     opCtx->recoveryUnit()->onCommit([]() {
         serverGlobalParams.featureCompatibility.setVersion(
-            ServerGlobalParams::FeatureCompatibility::Version::k34);
+            ServerGlobalParams::FeatureCompatibility::Version::kFullyDowngradedTo34);
     });
 }
 
@@ -388,7 +388,7 @@ void FeatureCompatibilityVersion::onDropCollection(OperationContext* opCtx) {
           << FeatureCompatibilityVersionCommandParser::kVersion34;
     opCtx->recoveryUnit()->onCommit([]() {
         serverGlobalParams.featureCompatibility.setVersion(
-            ServerGlobalParams::FeatureCompatibility::Version::k34);
+            ServerGlobalParams::FeatureCompatibility::Version::kFullyDowngradedTo34);
     });
 }
 
@@ -452,11 +452,11 @@ public:
         if (serverGlobalParams.featureCompatibility.isFullyUpgradedTo36()) {
             b.append(name,
                      FeatureCompatibilityVersion::toString(
-                         ServerGlobalParams::FeatureCompatibility::Version::k36));
+                         ServerGlobalParams::FeatureCompatibility::Version::kFullyUpgradedTo36));
         } else {
             b.append(name,
                      FeatureCompatibilityVersion::toString(
-                         ServerGlobalParams::FeatureCompatibility::Version::k34));
+                         ServerGlobalParams::FeatureCompatibility::Version::kFullyDowngradedTo34));
         }
     }
 
diff --git a/src/mongo/db/commands/feature_compatibility_version.h b/src/mongo/db/commands/feature_compatibility_version.h
index dd578f9dd6a..e1598756919 100644
--- a/src/mongo/db/commands/feature_compatibility_version.h
+++ b/src/mongo/db/commands/feature_compatibility_version.h
@@ -70,15 +70,15 @@ public:
 
     static StringData toString(ServerGlobalParams::FeatureCompatibility::Version version) {
         switch (version) {
-            case ServerGlobalParams::FeatureCompatibility::Version::k36:
+            case ServerGlobalParams::FeatureCompatibility::Version::kFullyUpgradedTo36:
                 return FeatureCompatibilityVersionCommandParser::kVersion36;
-            case ServerGlobalParams::FeatureCompatibility::Version::k34:
+            case ServerGlobalParams::FeatureCompatibility::Version::kFullyDowngradedTo34:
                 return FeatureCompatibilityVersionCommandParser::kVersion34;
             case ServerGlobalParams::FeatureCompatibility::Version::kUpgradingTo36:
                 return FeatureCompatibilityVersionCommandParser::kVersionUpgradingTo36;
             case ServerGlobalParams::FeatureCompatibility::Version::kDowngradingTo34:
                 return FeatureCompatibilityVersionCommandParser::kVersionDowngradingTo34;
-            case ServerGlobalParams::FeatureCompatibility::Version::kUnset:
+            case ServerGlobalParams::FeatureCompatibility::Version::kUnsetDefault34Behavior:
                 return FeatureCompatibilityVersionCommandParser::kVersionUnset;
             default:
                 MONGO_UNREACHABLE;
diff --git a/src/mongo/db/keys_collection_cache_reader_and_updater_test.cpp b/src/mongo/db/keys_collection_cache_reader_and_updater_test.cpp
index 0dbaacad30a..83ea491a30c 100644
--- a/src/mongo/db/keys_collection_cache_reader_and_updater_test.cpp
+++ b/src/mongo/db/keys_collection_cache_reader_and_updater_test.cpp
@@ -55,7 +55,7 @@ protected:
         ConfigServerTestFixture::setUp();
 
         serverGlobalParams.featureCompatibility.setVersion(
-            ServerGlobalParams::FeatureCompatibility::Version::k36);
+            ServerGlobalParams::FeatureCompatibility::Version::kFullyUpgradedTo36);
         serverGlobalParams.validateFeaturesAsMaster.store(true);
 
         auto clockSource = stdx::make_unique<ClockSourceMock>();
@@ -458,7 +458,7 @@ TEST_F(CacheUpdaterTest, ShouldNotCreateKeysWithDisableKeyGenerationFailPoint) {
 
 TEST_F(CacheUpdaterTest, ShouldNotCreateNewKeysInFeatureCompatiblityVersion34) {
     serverGlobalParams.featureCompatibility.setVersion(
-        ServerGlobalParams::FeatureCompatibility::Version::k34);
+        ServerGlobalParams::FeatureCompatibility::Version::kFullyDowngradedTo34);
 
     KeysCollectionCacheReaderAndUpdater updater("dummy", catalogClient(), Seconds(5));
 
@@ -475,7 +475,7 @@ TEST_F(CacheUpdaterTest, ShouldNotCreateNewKeysInFeatureCompatiblityVersion34) {
 
     // Increase the feature compatibility version and verify keys are found after refresh.
     serverGlobalParams.featureCompatibility.setVersion(
-        ServerGlobalParams::FeatureCompatibility::Version::k36);
+        ServerGlobalParams::FeatureCompatibility::Version::kFullyUpgradedTo36);
 
     {
         auto keyStatus = updater.refresh(operationContext());
diff --git a/src/mongo/db/keys_collection_manager_sharding_test.cpp b/src/mongo/db/keys_collection_manager_sharding_test.cpp
index 8006238a8c3..0ba9588ce50 100644
--- a/src/mongo/db/keys_collection_manager_sharding_test.cpp
+++ b/src/mongo/db/keys_collection_manager_sharding_test.cpp
@@ -60,7 +60,7 @@ protected:
         ConfigServerTestFixture::setUp();
 
         serverGlobalParams.featureCompatibility.setVersion(
-            ServerGlobalParams::FeatureCompatibility::Version::k36);
+            ServerGlobalParams::FeatureCompatibility::Version::kFullyUpgradedTo36);
         serverGlobalParams.validateFeaturesAsMaster.store(true);
 
         auto clockSource = stdx::make_unique<ClockSourceMock>();
@@ -376,7 +376,7 @@ TEST_F(KeysManagerShardedTest, HasSeenKeysIsFalseUntilKeysAreFound) {
 
 TEST_F(KeysManagerShardedTest, ShouldNotReturnKeysInFeatureCompatibilityVersion34) {
     serverGlobalParams.featureCompatibility.setVersion(
-        ServerGlobalParams::FeatureCompatibility::Version::k34);
+        ServerGlobalParams::FeatureCompatibility::Version::kFullyDowngradedTo34);
 
     keyManager()->startMonitoring(getServiceContext());
     keyManager()->enableKeyGenerator(operationContext(), true);
diff --git a/src/mongo/db/logical_session_cache_test.cpp b/src/mongo/db/logical_session_cache_test.cpp
index 5e96394a515..d7f8f064f26 100644
--- a/src/mongo/db/logical_session_cache_test.cpp
+++ b/src/mongo/db/logical_session_cache_test.cpp
@@ -67,7 +67,7 @@ public:
     LogicalSessionCacheTest()
         : _service(std::make_shared<MockServiceLiasonImpl>()),
           _sessions(std::make_shared<MockSessionsCollectionImpl>()),
-          _fcv(EnsureFCV::Version::k36) {}
+          _fcv(EnsureFCV::Version::kFullyUpgradedTo36) {}
 
     void setUp() override {
         auto localManagerState = stdx::make_unique<AuthzManagerExternalStateMock>();
diff --git a/src/mongo/db/logical_session_id_test.cpp b/src/mongo/db/logical_session_id_test.cpp
index 9e9e828372b..2f761f3d302 100644
--- a/src/mongo/db/logical_session_id_test.cpp
+++ b/src/mongo/db/logical_session_id_test.cpp
@@ -71,7 +71,7 @@ public:
 
     void setUp() {
         serverGlobalParams.featureCompatibility.setVersion(
-            ServerGlobalParams::FeatureCompatibility::Version::k36);
+            ServerGlobalParams::FeatureCompatibility::Version::kFullyUpgradedTo36);
         session = transportLayer.createSession();
         client = serviceContext.makeClient("testClient", session);
         RestrictionEnvironment::set(
diff --git a/src/mongo/db/logical_time_validator_test.cpp b/src/mongo/db/logical_time_validator_test.cpp
index 92ddc3213ef..1bb46ba0802 100644
--- a/src/mongo/db/logical_time_validator_test.cpp
+++ b/src/mongo/db/logical_time_validator_test.cpp
@@ -62,7 +62,7 @@ protected:
         ConfigServerTestFixture::setUp();
 
         serverGlobalParams.featureCompatibility.setVersion(
-            ServerGlobalParams::FeatureCompatibility::Version::k36);
+            ServerGlobalParams::FeatureCompatibility::Version::kFullyUpgradedTo36);
         serverGlobalParams.validateFeaturesAsMaster.store(true);
 
         auto clockSource = stdx::make_unique<ClockSourceMock>();
diff --git a/src/mongo/db/pipeline/document_source_change_stream_test.cpp b/src/mongo/db/pipeline/document_source_change_stream_test.cpp
index f59545111f0..78fec65f9cc 100644
--- a/src/mongo/db/pipeline/document_source_change_stream_test.cpp
+++ b/src/mongo/db/pipeline/document_source_change_stream_test.cpp
@@ -77,7 +77,7 @@ class ChangeStreamStageTestNoSetup : public AggregationContextFixture {
 public:
     ChangeStreamStageTestNoSetup() : ChangeStreamStageTestNoSetup(nss) {}
     ChangeStreamStageTestNoSetup(NamespaceString nsString)
-        : AggregationContextFixture(nsString), _ensureFCV(EnsureFCV::Version::k36) {}
+        : AggregationContextFixture(nsString), _ensureFCV(EnsureFCV::Version::kFullyUpgradedTo36) {}
 
 private:
     EnsureFCV _ensureFCV;
diff --git a/src/mongo/db/pipeline/document_source_lookup_test.cpp b/src/mongo/db/pipeline/document_source_lookup_test.cpp
index 54e58c25b58..3ad105a4414 100644
--- a/src/mongo/db/pipeline/document_source_lookup_test.cpp
+++ b/src/mongo/db/pipeline/document_source_lookup_test.cpp
@@ -239,7 +239,7 @@ TEST_F(DocumentSourceLookUpTest, LiteParsedDocumentSourceLookupContainsExpectedN
 
 TEST_F(ReplDocumentSourceLookUpTest, RejectsPipelineWithChangeStreamStage) {
     // Temporarily set FCV to 3.6 for $changeStream.
-    EnsureFCV ensureFCV(EnsureFCV::Version::k36);
+    EnsureFCV ensureFCV(EnsureFCV::Version::kFullyUpgradedTo36);
 
     auto expCtx = getExpCtx();
     NamespaceString fromNs("test", "coll");
@@ -257,7 +257,7 @@ TEST_F(ReplDocumentSourceLookUpTest, RejectsPipelineWithChangeStreamStage) {
 
 TEST_F(ReplDocumentSourceLookUpTest, RejectsSubPipelineWithChangeStreamStage) {
     // Temporarily set FCV to 3.6 for $changeStream.
-    EnsureFCV ensureFCV(EnsureFCV::Version::k36);
+    EnsureFCV ensureFCV(EnsureFCV::Version::kFullyUpgradedTo36);
 
     auto expCtx = getExpCtx();
     NamespaceString fromNs("test", "coll");
diff --git a/src/mongo/db/pipeline/pipeline_test.cpp b/src/mongo/db/pipeline/pipeline_test.cpp
index 4418e9dc9a4..cf90fd4e77e 100644
--- a/src/mongo/db/pipeline/pipeline_test.cpp
+++ b/src/mongo/db/pipeline/pipeline_test.cpp
@@ -1329,7 +1329,7 @@ TEST(PipelineOptimizationTest, MatchOnFmodShouldSwapWithAdjacentStage) {
 }
 
 TEST(PipelineOptimizationTest, ChangeStreamLookupSwapsWithIndependentMatch) {
-    EnsureFCV ensureFCV(EnsureFCV::Version::k36);
+    EnsureFCV ensureFCV(EnsureFCV::Version::kFullyUpgradedTo36);
     QueryTestServiceContext testServiceContext;
     auto opCtx = testServiceContext.makeOperationContext();
 
@@ -1355,7 +1355,7 @@ TEST(PipelineOptimizationTest, ChangeStreamLookupSwapsWithIndependentMatch) {
 }
 
 TEST(PipelineOptimizationTest, ChangeStreamLookupDoesNotSwapWithMatchOnPostImage) {
-    EnsureFCV ensureFCV(EnsureFCV::Version::k36);
+    EnsureFCV ensureFCV(EnsureFCV::Version::kFullyUpgradedTo36);
     QueryTestServiceContext testServiceContext;
     auto opCtx = testServiceContext.makeOperationContext();
 
@@ -1987,7 +1987,7 @@ TEST_F(PipelineInitialSourceNSTest, AggregateOneNSValidForFacetPipelineRegardles
 }
 
 TEST_F(PipelineInitialSourceNSTest, ChangeStreamIsValidAsFirstStage) {
-    EnsureFCV ensureFCV(EnsureFCV::Version::k36);
+    EnsureFCV ensureFCV(EnsureFCV::Version::kFullyUpgradedTo36);
     const std::vector<BSONObj> rawPipeline = {fromjson("{$changeStream: {}}")};
     auto ctx = getExpCtx();
     setMockReplicationCoordinatorOnOpCtx(ctx->opCtx);
@@ -1996,7 +1996,7 @@ TEST_F(PipelineInitialSourceNSTest, ChangeStreamIsValidAsFirstStage) {
 }
 
 TEST_F(PipelineInitialSourceNSTest, ChangeStreamIsNotValidIfNotFirstStage) {
-    EnsureFCV ensureFCV(EnsureFCV::Version::k36);
+    EnsureFCV ensureFCV(EnsureFCV::Version::kFullyUpgradedTo36);
     const std::vector<BSONObj> rawPipeline = {fromjson("{$match: {custom: 'filter'}}"),
                                               fromjson("{$changeStream: {}}")};
     auto ctx = getExpCtx();
@@ -2007,7 +2007,7 @@ TEST_F(PipelineInitialSourceNSTest, ChangeStreamIsNotValidIfNotFirstStage) {
 }
 
 TEST_F(PipelineInitialSourceNSTest, ChangeStreamIsNotValidIfNotFirstStageInFacet) {
-    EnsureFCV ensureFCV(EnsureFCV::Version::k36);
+    EnsureFCV ensureFCV(EnsureFCV::Version::kFullyUpgradedTo36);
     const std::vector<BSONObj> rawPipeline = {fromjson("{$match: {custom: 'filter'}}"),
                                               fromjson("{$changeStream: {}}")};
     auto ctx = getExpCtx();
diff --git a/src/mongo/db/repl/idempotency_test.cpp b/src/mongo/db/repl/idempotency_test.cpp
index 5801b07a0cb..d001f704508 100644
--- a/src/mongo/db/repl/idempotency_test.cpp
+++ b/src/mongo/db/repl/idempotency_test.cpp
@@ -194,13 +194,13 @@ void RandomizedIdempotencyTest::runIdempotencyTestCase() {
 
 TEST_F(RandomizedIdempotencyTest, CheckUpdateSequencesAreIdempotentWhenFCV34) {
     serverGlobalParams.featureCompatibility.setVersion(
-        ServerGlobalParams::FeatureCompatibility::Version::k34);
+        ServerGlobalParams::FeatureCompatibility::Version::kFullyDowngradedTo34);
     runIdempotencyTestCase();
 }
 
 TEST_F(RandomizedIdempotencyTest, CheckUpdateSequencesAreIdempotentWhenFCV36) {
     serverGlobalParams.featureCompatibility.setVersion(
-        ServerGlobalParams::FeatureCompatibility::Version::k36);
+        ServerGlobalParams::FeatureCompatibility::Version::kFullyUpgradedTo36);
     runIdempotencyTestCase();
 }
 
diff --git a/src/mongo/db/repl/oplog_fetcher_test.cpp b/src/mongo/db/repl/oplog_fetcher_test.cpp
index 4b63bdc475f..35af92df177 100644
--- a/src/mongo/db/repl/oplog_fetcher_test.cpp
+++ b/src/mongo/db/repl/oplog_fetcher_test.cpp
@@ -281,7 +281,7 @@ TEST_F(OplogFetcherTest, FindQueryHasNoReadconcernIfTermUninitialized) {
 }
 
 TEST_F(OplogFetcherTest, FindQueryHasAfterOpTimeWithFeatureCompatibilityVersion34) {
-    EnsureFCV ensureFCV(EnsureFCV::Version::k34);
+    EnsureFCV ensureFCV(EnsureFCV::Version::kFullyDowngradedTo34);
     ASSERT(!serverGlobalParams.featureCompatibility.isFullyUpgradedTo36());
     auto cmdObj = makeOplogFetcher(_createConfig(true))->getFindQuery_forTest();
     auto readConcernElem = cmdObj["readConcern"];
@@ -293,7 +293,7 @@ TEST_F(OplogFetcherTest, FindQueryHasAfterOpTimeWithFeatureCompatibilityVersion3
 }
 
 TEST_F(OplogFetcherTest, FindQueryHasAfterOpTimeWithFeatureCompatibilityVersion36) {
-    EnsureFCV ensureFCV(EnsureFCV::Version::k36);
+    EnsureFCV ensureFCV(EnsureFCV::Version::kFullyUpgradedTo36);
     ASSERT(serverGlobalParams.featureCompatibility.isFullyUpgradedTo36());
     auto cmdObj = makeOplogFetcher(_createConfig(true))->getFindQuery_forTest();
     auto readConcernElem = cmdObj["readConcern"];
diff --git a/src/mongo/db/repl/replication_consistency_markers_impl_test.cpp b/src/mongo/db/repl/replication_consistency_markers_impl_test.cpp
index 105613c516f..69a233379ed 100644
--- a/src/mongo/db/repl/replication_consistency_markers_impl_test.cpp
+++ b/src/mongo/db/repl/replication_consistency_markers_impl_test.cpp
@@ -317,7 +317,7 @@ TEST_F(ReplicationConsistencyMarkersTest, OplogTruncateAfterPointUpgrade) {
 
     // Set the feature compatibility version to 3.6.
     serverGlobalParams.featureCompatibility.setVersion(
-        ServerGlobalParams::FeatureCompatibility::Version::k36);
+        ServerGlobalParams::FeatureCompatibility::Version::kFullyUpgradedTo36);
 
     // Check that we see no oplog truncate after point in FCV 3.6.
     ASSERT(consistencyMarkers.getOplogTruncateAfterPoint(opCtx).isNull());
@@ -325,7 +325,7 @@ TEST_F(ReplicationConsistencyMarkersTest, OplogTruncateAfterPointUpgrade) {
 
     // Set the feature compatibility version to 3.4.
     serverGlobalParams.featureCompatibility.setVersion(
-        ServerGlobalParams::FeatureCompatibility::Version::k34);
+        ServerGlobalParams::FeatureCompatibility::Version::kFullyDowngradedTo34);
 
     // Check that we see the old oplog delete from point in FCV 3.4.
     ASSERT_EQ(consistencyMarkers.getOplogTruncateAfterPoint(opCtx), time1);
@@ -348,7 +348,7 @@ TEST_F(ReplicationConsistencyMarkersTest, OplogTruncateAfterPointUpgrade) {
 
     // Set the feature compatibility version to 3.6.
     serverGlobalParams.featureCompatibility.setVersion(
-        ServerGlobalParams::FeatureCompatibility::Version::k36);
+        ServerGlobalParams::FeatureCompatibility::Version::kFullyUpgradedTo36);
 
     // Check that after removing the old oplog delete from point, that we do not see the oplog
     // truncate after point in FCV 3.6.
diff --git a/src/mongo/db/repl/replication_info.cpp b/src/mongo/db/repl/replication_info.cpp
index f27e1608e73..3391dc3aa19 100644
--- a/src/mongo/db/repl/replication_info.cpp
+++ b/src/mongo/db/repl/replication_info.cpp
@@ -364,7 +364,7 @@ public:
         // connect.
         if (internalClientElement &&
             serverGlobalParams.featureCompatibility.getVersion() ==
-                ServerGlobalParams::FeatureCompatibility::Version::k36) {
+                ServerGlobalParams::FeatureCompatibility::Version::kFullyUpgradedTo36) {
             result.append("minWireVersion", WireSpec::instance().incoming.maxWireVersion);
         } else {
             result.append("minWireVersion", WireSpec::instance().incoming.minWireVersion);
diff --git a/src/mongo/db/repl/sync_tail_test.cpp b/src/mongo/db/repl/sync_tail_test.cpp
index d1337ca5b20..e964e291b50 100644
--- a/src/mongo/db/repl/sync_tail_test.cpp
+++ b/src/mongo/db/repl/sync_tail_test.cpp
@@ -1666,7 +1666,7 @@ TEST_F(SyncTailTest, UpdateToFCVCollectionBesidesFCVDocumentSucceedsInRecovering
 TEST_F(SyncTailTest, UpgradeWithNoUUIDFailsInSecondary) {
     // Set fCV to 3.4 so the node does not create a UUID for the collection.
     serverGlobalParams.featureCompatibility.setVersion(
-        ServerGlobalParams::FeatureCompatibility::Version::k34);
+        ServerGlobalParams::FeatureCompatibility::Version::kFullyDowngradedTo34);
 
     auto fcvNS = NamespaceString(FeatureCompatibilityVersion::kCollection);
     ::mongo::repl::createCollection(_opCtx.get(), fcvNS, CollectionOptions());
diff --git a/src/mongo/db/repl/sync_tail_test_fixture.cpp b/src/mongo/db/repl/sync_tail_test_fixture.cpp
index 0026f658fad..070f76dfc8c 100644
--- a/src/mongo/db/repl/sync_tail_test_fixture.cpp
+++ b/src/mongo/db/repl/sync_tail_test_fixture.cpp
@@ -92,7 +92,7 @@ void SyncTailTest::setUp() {
     _incOps = [this]() { _opsApplied++; };
 
     serverGlobalParams.featureCompatibility.setVersion(
-        ServerGlobalParams::FeatureCompatibility::Version::k36);
+        ServerGlobalParams::FeatureCompatibility::Version::kFullyUpgradedTo36);
 }
 
 void SyncTailTest::tearDown() {
diff --git a/src/mongo/db/s/session_catalog_migration_destination_test.cpp b/src/mongo/db/s/session_catalog_migration_destination_test.cpp
index c7ea51be2c1..99ee8eae5d8 100644
--- a/src/mongo/db/s/session_catalog_migration_destination_test.cpp
+++ b/src/mongo/db/s/session_catalog_migration_destination_test.cpp
@@ -84,7 +84,7 @@ class SessionCatalogMigrationDestinationTest : public ShardingMongodTestFixture
 public:
     void setUp() override {
         serverGlobalParams.featureCompatibility.setVersion(
-            ServerGlobalParams::FeatureCompatibility::Version::k36);
+            ServerGlobalParams::FeatureCompatibility::Version::kFullyUpgradedTo36);
         serverGlobalParams.clusterRole = ClusterRole::ShardServer;
         ShardingMongodTestFixture::setUp();
 
diff --git a/src/mongo/db/server_options.h b/src/mongo/db/server_options.h
index 1263e272b06..1c84b5d33ad 100644
--- a/src/mongo/db/server_options.h
+++ b/src/mongo/db/server_options.h
@@ -152,38 +152,45 @@ struct ServerGlobalParams {
          *
          * The legal enum (and featureCompatiblityVersion document) states are:
          *
-         * k34
-         * (3.4, Unset) aka fully 3.4: Only 3.4 features are available, and new and existing storage
-         *                             engine entries use the 3.4 format
+         * kFullyDowngradedTo34
+         * (3.4, Unset): Only 3.4 features are available, and new and existing storage
+         *               engine entries use the 3.4 format
          *
          * kUpgradingTo36
-         * (3.4, 3.6) aka upgrading: Only 3.4 features are available, but new storage engine entries
-         *                           use the 3.6 format, and existing entries may have either the
-         *                           3.4 or 3.6 format
+         * (3.4, 3.6): Only 3.4 features are available, but new storage engine entries
+         *             use the 3.6 format, and existing entries may have either the
+         *             3.4 or 3.6 format
          *
-         * k36
-         * (3.6, Unset) aka fully 3.6: 3.6 features are available, and new and existing storage
-         *                             engine entries use the 3.6 format
+         * kFullyUpgradedTo36
+         * (3.6, Unset): 3.6 features are available, and new and existing storage
+         *               engine entries use the 3.6 format
          *
          * kDowngradingTo34
-         * (3.4, 3.4) aka downgrading: Only 3.4 features are available and new storage engine
-         *                             entries use the 3.4 format, but existing entries may have
-         *                             either the 3.4 or 3.6 format
+         * (3.4, 3.4): Only 3.4 features are available and new storage engine
+         *             entries use the 3.4 format, but existing entries may have
+         *             either the 3.4 or 3.6 format
          *
-         * kUnset
-         * (Unset, Unset) aka uninitialized: This is the case on startup before the fCV document is
-         *                                   loaded into memory. isVersionInitialized() will return
-         *                                   false, and getVersion() will return the default (k34).
+         * kUnsetDefault34Behavior
+         * (Unset, Unset): This is the case on startup before the fCV document is
+         *                 loaded into memory. isVersionInitialized() will return
+         *                 false, and getVersion() will return the default
+         *                 (kFullyDowngradedTo34).
          *
          */
-        enum class Version { k34, kUpgradingTo36, k36, kDowngradingTo34, kUnset };
+        enum class Version {
+            kFullyDowngradedTo34,
+            kUpgradingTo36,
+            kFullyUpgradedTo36,
+            kDowngradingTo34,
+            kUnsetDefault34Behavior
+        };
 
         /**
          * On startup, the featureCompatibilityVersion may not have been explicitly set yet. This
          * exposes the actual state of the featureCompatibilityVersion if it is uninitialized.
          */
         const bool isVersionInitialized() const {
-            return _version.load() != Version::kUnset;
+            return _version.load() != Version::kUnsetDefault34Behavior;
         }
 
         /**
@@ -192,29 +199,33 @@ struct ServerGlobalParams {
          */
         const Version getVersion() const {
             Version v = _version.load();
-            return (v == Version::kUnset) ? Version::k34 : v;
+            return (v == Version::kUnsetDefault34Behavior) ? Version::kFullyDowngradedTo34 : v;
         }
 
         void reset() {
-            _version.store(Version::k34);
+            _version.store(Version::kFullyDowngradedTo34);
         }
 
         void setVersion(Version version) {
             return _version.store(version);
         }
 
+        // Deprecated. Please use getVersion() directly instead.
         const bool isFullyUpgradedTo36() {
-            return (getVersion() == Version::k36);
+            return (getVersion() == Version::kFullyUpgradedTo36);
         }
 
+        // Deprecated. Please use getVersion() directly instead.
         const bool isUpgradingTo36() {
             return (getVersion() == Version::kUpgradingTo36);
         }
 
+        // Deprecated. Please use getVersion() directly instead.
         const bool isFullyDowngradedTo34() {
-            return (getVersion() == Version::k34);
+            return (getVersion() == Version::kFullyDowngradedTo34);
         }
 
+        // Deprecated. Please use getVersion() directly instead.
         const bool isDowngradingTo34() {
             return (getVersion() == Version::kDowngradingTo34);
         }
@@ -225,7 +236,7 @@ struct ServerGlobalParams {
         }
 
     private:
-        AtomicWord<Version> _version{Version::kUnset};
+        AtomicWord<Version> _version{Version::kUnsetDefault34Behavior};
 
     } featureCompatibility;
 
diff --git a/src/mongo/db/views/view_catalog_test.cpp b/src/mongo/db/views/view_catalog_test.cpp
index d26256cf40d..a989bd11e9b 100644
--- a/src/mongo/db/views/view_catalog_test.cpp
+++ b/src/mongo/db/views/view_catalog_test.cpp
@@ -180,7 +180,7 @@ TEST_F(ViewCatalogFixture, CreateViewOnDifferentDatabase) {
 
 // TODO SERVER-31588: Remove FCV 3.4 validation during the 3.7 development cycle.
 TEST_F(ViewCatalogFixture, CreateViewWith36FeaturesSucceedsUnder36FCV) {
-    EnsureFCV ensureFCV(EnsureFCV::Version::k36);
+    EnsureFCV ensureFCV(EnsureFCV::Version::kFullyUpgradedTo36);
 
     const NamespaceString viewOn("db.coll");
 
@@ -229,7 +229,7 @@ TEST_F(ViewCatalogFixture, CreateViewWith36FeaturesSucceedsUnder36FCV) {
 
 // TODO SERVER-31588: Remove FCV 3.4 validation during the 3.7 development cycle.
 TEST_F(ViewCatalogFixture, CreateViewWith36FeaturesFailsUnder34FCV) {
-    EnsureFCV ensureFCV(EnsureFCV::Version::k34);
+    EnsureFCV ensureFCV(EnsureFCV::Version::kFullyDowngradedTo34);
 
     const NamespaceString viewName("db.view");
     const NamespaceString viewOn("db.coll");
@@ -313,7 +313,7 @@ TEST_F(ViewCatalogFixture, CreateViewWithPipelineFailsOnInvalidStageName) {
 
 TEST_F(ReplViewCatalogFixture, CreateViewWithPipelineFailsOnIneligibleStage) {
     // Temporarily set the feature version to 3.6 for $changeStream.
-    EnsureFCV ensureFCV(EnsureFCV::Version::k36);
+    EnsureFCV ensureFCV(EnsureFCV::Version::kFullyUpgradedTo36);
 
     const NamespaceString viewName("db.view");
     const NamespaceString viewOn("db.coll");
@@ -489,7 +489,7 @@ TEST_F(ViewCatalogFixture, ModifyViewOnInvalidCollectionName) {
 
 TEST_F(ReplViewCatalogFixture, ModifyViewWithPipelineFailsOnIneligibleStage) {
     // Temporarily set the feature version to 3.6 for $changeStream.
-    EnsureFCV ensureFCV(EnsureFCV::Version::k36);
+    EnsureFCV ensureFCV(EnsureFCV::Version::kFullyUpgradedTo36);
 
     const NamespaceString viewName("db.view");
     const NamespaceString viewOn("db.coll");
diff --git a/src/mongo/dbtests/dbtests.cpp b/src/mongo/dbtests/dbtests.cpp
index 01076391f9f..4fd7416d9fa 100644
--- a/src/mongo/dbtests/dbtests.cpp
+++ b/src/mongo/dbtests/dbtests.cpp
@@ -122,7 +122,7 @@ int dbtestsMain(int argc, char** argv, char** envp) {
     mongo::dbtests::initWireSpec();
     mongo::runGlobalInitializersOrDie(argc, argv, envp);
     serverGlobalParams.featureCompatibility.setVersion(
-        ServerGlobalParams::FeatureCompatibility::Version::k36);
+        ServerGlobalParams::FeatureCompatibility::Version::kFullyUpgradedTo36);
     repl::ReplSettings replSettings;
     replSettings.setOplogSizeBytes(10 * 1024 * 1024);
     ServiceContext* service = getGlobalServiceContext();
diff --git a/src/mongo/s/server.cpp b/src/mongo/s/server.cpp
index aa28c33d81f..27ced948e06 100644
--- a/src/mongo/s/server.cpp
+++ b/src/mongo/s/server.cpp
@@ -486,7 +486,7 @@ MONGO_INITIALIZER_GENERAL(ForkServer, ("EndStartupOptionHandling"), ("default"))
 MONGO_INITIALIZER_WITH_PREREQUISITES(SetFeatureCompatibilityVersion36, ("EndStartupOptionStorage"))
 (InitializerContext* context) {
     mongo::serverGlobalParams.featureCompatibility.setVersion(
-        ServerGlobalParams::FeatureCompatibility::Version::k36);
+        ServerGlobalParams::FeatureCompatibility::Version::kFullyUpgradedTo36);
     return Status::OK();
 }
 
diff --git a/src/mongo/shell/dbshell.cpp b/src/mongo/shell/dbshell.cpp
index 72f9f060b0c..c3c30ed5b09 100644
--- a/src/mongo/shell/dbshell.cpp
+++ b/src/mongo/shell/dbshell.cpp
@@ -96,7 +96,7 @@ const auto kDefaultMongoURL = "mongodb://127.0.0.1:27017"_sd;
 MONGO_INITIALIZER_WITH_PREREQUISITES(SetFeatureCompatibilityVersion36, ("EndStartupOptionSetup"))
 (InitializerContext* context) {
     mongo::serverGlobalParams.featureCompatibility.setVersion(
-        ServerGlobalParams::FeatureCompatibility::Version::k36);
+        ServerGlobalParams::FeatureCompatibility::Version::kFullyUpgradedTo36);
     return Status::OK();
 }
 }  // namespace