SERVER-56366 Update FCV constants to recognize 5.1 as latest

21 files changed, 119 insertions, 115 deletions

diff --git a/src/mongo/db/catalog/collection_impl.cpp b/src/mongo/db/catalog/collection_impl.cpp
index 423cb137edc..5a54dd455e4 100644
--- a/src/mongo/db/catalog/collection_impl.cpp
+++ b/src/mongo/db/catalog/collection_impl.cpp
@@ -863,7 +863,7 @@ void CollectionImpl::_cappedDeleteAsNeeded(OperationContext* opCtx,
     }
 
     bool useOldCappedDeleteBehaviour = serverGlobalParams.featureCompatibility.isLessThan(
-        ServerGlobalParams::FeatureCompatibility::Version::kVersion50);
+        ServerGlobalParams::FeatureCompatibility::Version::kFullyDowngradedTo50);
 
     if (!useOldCappedDeleteBehaviour && !opCtx->isEnforcingConstraints()) {
         // With new capped delete behavior, secondaries only delete from capped collections via
diff --git a/src/mongo/db/commands/feature_compatibility_version_parser.cpp b/src/mongo/db/commands/feature_compatibility_version_parser.cpp
index 12f2fb3132d..087755d9186 100644
--- a/src/mongo/db/commands/feature_compatibility_version_parser.cpp
+++ b/src/mongo/db/commands/feature_compatibility_version_parser.cpp
@@ -69,8 +69,12 @@ FeatureCompatibilityParams::Version FeatureCompatibilityVersionParser::parseVers
 
 FeatureCompatibilityParams::Version FeatureCompatibilityVersionParser::parseVersionForFeatureFlags(
     StringData versionString) {
+    if (versionString == kVersion51) {
+        return FeatureCompatibilityParams::Version::kVersion51;
+    }
+
     if (versionString == kVersion50) {
-        return FeatureCompatibilityParams::Version::kVersion50;
+        return FeatureCompatibilityParams::Version::kFullyDowngradedTo50;
     }
 
     if (versionString == kVersion49) {
@@ -111,7 +115,11 @@ StringData FeatureCompatibilityVersionParser::serializeVersion(
 
 StringData FeatureCompatibilityVersionParser::serializeVersionForFeatureFlags(
     FeatureCompatibilityParams::Version version) {
-    if (version == FeatureCompatibilityParams::Version::kVersion50) {
+    if (version == FeatureCompatibilityParams::Version::kVersion51) {
+        return kVersion51;
+    }
+
+    if (version == FeatureCompatibilityParams::Version::kFullyDowngradedTo50) {
         return kVersion50;
     }
 
diff --git a/src/mongo/db/commands/feature_compatibility_version_parser.h b/src/mongo/db/commands/feature_compatibility_version_parser.h
index e54e16a71f9..b20cb3a1b01 100644
--- a/src/mongo/db/commands/feature_compatibility_version_parser.h
+++ b/src/mongo/db/commands/feature_compatibility_version_parser.h
@@ -41,41 +41,29 @@ using FeatureCompatibilityParams = ServerGlobalParams::FeatureCompatibility;
  */
 class FeatureCompatibilityVersionParser {
 public:
-    static constexpr StringData kVersion44 = "4.4"_sd;
-    static constexpr StringData kVersion47 = "4.7"_sd;
-    static constexpr StringData kVersion48 = "4.8"_sd;
-    static constexpr StringData kVersionDowngradingFrom47To44 = "downgrading from 4.7 to 4.4"_sd;
-    static constexpr StringData kVersionDowngradingFrom48To44 = "downgrading from 4.8 to 4.4"_sd;
-    static constexpr StringData kVersionDowngradingFrom48To47 = "downgrading from 4.8 to 4.7"_sd;
-    static constexpr StringData kVersionUpgradingFrom44To47 = "upgrading from 4.4 to 4.7"_sd;
-    static constexpr StringData kVersionUpgradingFrom47To48 = "upgrading from 4.7 to 4.8"_sd;
-    static constexpr StringData kVersionUpgradingFrom44To48 = "upgrading from 4.4 to 4.8"_sd;
-    static constexpr StringData kVersionDowngradingFrom49To44 = "downgrading from 4.9 to 4.4"_sd;
-    static constexpr StringData kVersionUpgradingFrom44To49 = "upgrading from 4.4 to 4.9"_sd;
-    static constexpr StringData kVersionDowngradingFrom49To48 = "downgrading from 4.9 to 4.8"_sd;
-    static constexpr StringData kVersionUpgradingFrom48To49 = "upgrading from 4.8 to 4.9"_sd;
-    static constexpr StringData kVersion49 = "4.9"_sd;
-    static constexpr StringData kVersionDowngradingFrom50To44 = "downgrading from 5.0 to 4.4"_sd;
-    static constexpr StringData kVersionUpgradingFrom44To50 = "upgrading from 4.4 to 5.0"_sd;
-    static constexpr StringData kVersionDowngradingFrom50To49 = "downgrading from 5.0 to 4.9"_sd;
-    static constexpr StringData kVersionUpgradingFrom49To50 = "upgrading from 4.9 to 5.0"_sd;
+    static constexpr StringData kVersion44 = "4.4"_sd;  // Remove once old feature flags are deleted
+    static constexpr StringData kVersion47 = "4.7"_sd;  // Remove once old feature flags are deleted
+    static constexpr StringData kVersion48 = "4.8"_sd;  // Remove once old feature flags are deleted
+    static constexpr StringData kVersion49 = "4.9"_sd;  // Remove once old feature flags are deleted
     static constexpr StringData kVersion50 = "5.0"_sd;
+    static constexpr StringData kVersion51 = "5.1"_sd;
+    static constexpr StringData kVersionDowngradingFrom51To50 = "downgrading from 5.1 to 5.0"_sd;
+    static constexpr StringData kVersionUpgradingFrom50To51 = "upgrading from 5.0 to 5.1"_sd;
     static constexpr StringData kVersionUnset = "Unset"_sd;
 
     static constexpr StringData kParameterName = "featureCompatibilityVersion"_sd;
 
-    static constexpr StringData kLastLTS = kVersion44;
-    static constexpr StringData kLastContinuous = kVersion49;
-    static constexpr StringData kLatest = kVersion50;
-    static constexpr StringData kUpgradingFromLastLTSToLatest = kVersionUpgradingFrom44To50;
-    static constexpr StringData kUpgradingFromLastContinuousToLatest = kVersionUpgradingFrom49To50;
+    static constexpr StringData kLastLTS = kVersion50;
+    static constexpr StringData kLastContinuous = kVersion50;
+    static constexpr StringData kLatest = kVersion51;
+    static constexpr StringData kUpgradingFromLastLTSToLatest = kVersionUpgradingFrom50To51;
+    static constexpr StringData kUpgradingFromLastContinuousToLatest = kVersionUpgradingFrom50To51;
     // kVersionUpgradingFromLastLTSToLastContinuous should assigned kVersionUnset when kLastLTS and
     // kLastContinuous are equal.
-    static constexpr StringData kVersionUpgradingFromLastLTSToLastContinuous =
-        kVersionUpgradingFrom44To49;
-    static constexpr StringData kDowngradingFromLatestToLastLTS = kVersionDowngradingFrom50To44;
+    static constexpr StringData kVersionUpgradingFromLastLTSToLastContinuous = kVersionUnset;
+    static constexpr StringData kDowngradingFromLatestToLastLTS = kVersionDowngradingFrom51To50;
     static constexpr StringData kDowngradingFromLatestToLastContinuous =
-        kVersionDowngradingFrom50To49;
+        kVersionDowngradingFrom51To50;
 
     // Used to verify that FCV values in 'admin.system.version' are valid and equal to one of
     // { lastLTS, lastContinuous, latest }.
@@ -106,7 +94,7 @@ public:
      * Useful for message logging.
      */
     static StringData toString(FeatureCompatibilityParams::Version version) {
-        if (version == FeatureCompatibilityParams::Version::kUnsetDefault44Behavior) {
+        if (version == FeatureCompatibilityParams::Version::kUnsetDefault50Behavior) {
             return kVersionUnset;
         } else if (version == FeatureCompatibilityParams::kLastLTS) {
             return kLastLTS;
diff --git a/src/mongo/db/commands/find_and_modify.cpp b/src/mongo/db/commands/find_and_modify.cpp
index b7066244fcc..9222576fbe7 100644
--- a/src/mongo/db/commands/find_and_modify.cpp
+++ b/src/mongo/db/commands/find_and_modify.cpp
@@ -488,7 +488,7 @@ write_ops::FindAndModifyCommandReply CmdFindAndModify::Invocation::writeConflict
     if (updateResult.containsDotsAndDollarsField &&
         serverGlobalParams.featureCompatibility.isVersionInitialized() &&
         serverGlobalParams.featureCompatibility.isGreaterThanOrEqualTo(
-            FeatureCompatibilityParams::Version::kVersion50)) {
+            FeatureCompatibilityParams::Version::kFullyDowngradedTo50)) {
         // If it's an upsert, increment 'inserts' metric, otherwise increment 'updates'.
         dotsAndDollarsFieldsCounters.incrementForUpsert(!updateResult.upsertedId.isEmpty());
     }
diff --git a/src/mongo/db/commands/set_feature_compatibility_version_command.cpp b/src/mongo/db/commands/set_feature_compatibility_version_command.cpp
index d3b7a39728d..c226e06a1af 100644
--- a/src/mongo/db/commands/set_feature_compatibility_version_command.cpp
+++ b/src/mongo/db/commands/set_feature_compatibility_version_command.cpp
@@ -566,7 +566,7 @@ private:
 
         if (serverGlobalParams.clusterRole == ClusterRole::ConfigServer) {
             // TODO SERVER-53283: This block can removed once 5.0 becomes last-lts.
-            if (requestedVersion >= FeatureCompatibility::Version::kVersion50) {
+            if (requestedVersion == FeatureCompatibility::Version::kFullyDowngradedTo50) {
                 ShardingCatalogManager::get(opCtx)->upgradeMetadataFor50Phase1(opCtx);
             }
 
@@ -580,7 +580,7 @@ private:
                     opCtx, CommandHelpers::appendMajorityWriteConcern(requestPhase2.toBSON({}))));
 
             // TODO SERVER-53283: This block can removed once 5.0 becomes last-lts.
-            if (requestedVersion >= FeatureCompatibility::Version::kVersion50) {
+            if (requestedVersion == FeatureCompatibility::Version::kFullyDowngradedTo50) {
                 ShardingCatalogManager::get(opCtx)->upgradeMetadataFor50Phase2(opCtx);
             }
 
@@ -685,7 +685,7 @@ private:
             abortAllReshardCollection(opCtx);
 
             // TODO SERVER-53283: This block can removed once 5.0 becomes last-lts.
-            if (requestedVersion < FeatureCompatibility::Version::kVersion50) {
+            if (requestedVersion < FeatureCompatibility::Version::kFullyDowngradedTo50) {
                 ShardingCatalogManager::get(opCtx)->downgradeMetadataToPre50Phase1(opCtx);
 
                 // TODO: SERVER-55912 remove after 5.0 becomes last-lts.
@@ -702,12 +702,12 @@ private:
                     opCtx, CommandHelpers::appendMajorityWriteConcern(requestPhase2.toBSON({}))));
 
             // TODO SERVER-53283: This block can removed once 5.0 becomes last-lts.
-            if (requestedVersion < FeatureCompatibility::Version::kVersion50) {
+            if (requestedVersion < FeatureCompatibility::Version::kFullyDowngradedTo50) {
                 ShardingCatalogManager::get(opCtx)->downgradeMetadataToPre50Phase2(opCtx);
             }
         } else if (serverGlobalParams.clusterRole == ClusterRole::ShardServer) {
             // TODO: SERVER-55912 remove after 5.0 becomes last-lts.
-            if (requestedVersion < FeatureCompatibility::Version::kVersion50) {
+            if (requestedVersion < FeatureCompatibility::Version::kFullyDowngradedTo50) {
                 dropReshardingCollectionsOnShard(opCtx);
             }
         }
diff --git a/src/mongo/db/exec/upsert_stage.cpp b/src/mongo/db/exec/upsert_stage.cpp
index 1b352fc4d38..20904ae1909 100644
--- a/src/mongo/db/exec/upsert_stage.cpp
+++ b/src/mongo/db/exec/upsert_stage.cpp
@@ -279,7 +279,7 @@ void UpsertStage::_assertDocumentToBeInsertedIsValid(const mb::Document& documen
         bool allowTopLevelDollarPrefixes =
             serverGlobalParams.featureCompatibility.isVersionInitialized() &&
             serverGlobalParams.featureCompatibility.isGreaterThanOrEqualTo(
-                FeatureCompatibilityParams::Version::kVersion50);
+                FeatureCompatibilityParams::Version::kFullyDowngradedTo50);
         storage_validation::storageValid(document,
                                          allowTopLevelDollarPrefixes,
                                          true, /* Should validate for storage */
diff --git a/src/mongo/db/matcher/expression_parser.cpp b/src/mongo/db/matcher/expression_parser.cpp
index 3a2a1bc63b6..32a32d66683 100644
--- a/src/mongo/db/matcher/expression_parser.cpp
+++ b/src/mongo/db/matcher/expression_parser.cpp
@@ -296,7 +296,7 @@ StatusWithMatchExpression parse(const BSONObj& obj,
                 const auto dotsAndDollarsHint =
                     serverGlobalParams.featureCompatibility.isVersionInitialized() &&
                         serverGlobalParams.featureCompatibility.isGreaterThanOrEqualTo(
-                            FeatureCompatibilityParams::Version::kVersion50)
+                            FeatureCompatibilityParams::Version::kFullyDowngradedTo50)
                     ? ". If you have a field name that starts with a '$' symbol, consider using "
                       "$getField or $setField."
                     : "";
diff --git a/src/mongo/db/ops/insert.cpp b/src/mongo/db/ops/insert.cpp
index 44f22c877d5..e0accaac460 100644
--- a/src/mongo/db/ops/insert.cpp
+++ b/src/mongo/db/ops/insert.cpp
@@ -112,7 +112,7 @@ StatusWith<BSONObj> fixDocumentForInsert(OperationContext* opCtx,
             if (fieldName[0] == '$') {
                 if (!serverGlobalParams.featureCompatibility.isVersionInitialized() ||
                     !serverGlobalParams.featureCompatibility.isGreaterThanOrEqualTo(
-                        FeatureCompatibilityParams::Version::kVersion50)) {
+                        FeatureCompatibilityParams::Version::kFullyDowngradedTo50)) {
                     return StatusWith<BSONObj>(ErrorCodes::BadValue,
                                                str::stream()
                                                    << "Document can't have $ prefixed field names: "
diff --git a/src/mongo/db/pipeline/accumulator_sum.cpp b/src/mongo/db/pipeline/accumulator_sum.cpp
index b25fe3939ae..8322048df55 100644
--- a/src/mongo/db/pipeline/accumulator_sum.cpp
+++ b/src/mongo/db/pipeline/accumulator_sum.cpp
@@ -50,7 +50,7 @@ REGISTER_ACCUMULATOR(sum, genericParseSingleExpressionAccumulator<AccumulatorSum
 REGISTER_STABLE_EXPRESSION(sum, ExpressionFromAccumulator<AccumulatorSum>::parse);
 REGISTER_REMOVABLE_WINDOW_FUNCTION(sum, AccumulatorSum, WindowFunctionSum);
 REGISTER_ACCUMULATOR_WITH_MIN_VERSION(
-    count, parseCountAccumulator, ServerGlobalParams::FeatureCompatibility::Version::kVersion50);
+    count, parseCountAccumulator, ServerGlobalParams::FeatureCompatibility::Version::kVersion51);
 
 REGISTER_WINDOW_FUNCTION(count, window_function::parseCountWindowFunction);
 
diff --git a/src/mongo/db/pipeline/document_source_set_window_fields.cpp b/src/mongo/db/pipeline/document_source_set_window_fields.cpp
index ff728f14349..eaee99d5b82 100644
--- a/src/mongo/db/pipeline/document_source_set_window_fields.cpp
+++ b/src/mongo/db/pipeline/document_source_set_window_fields.cpp
@@ -78,14 +78,14 @@ REGISTER_DOCUMENT_SOURCE_WITH_MIN_VERSION(
     LiteParsedDocumentSourceDefault::parse,
     document_source_set_window_fields::createFromBson,
     AllowedWithApiStrict::kNeverInVersion1,
-    ServerGlobalParams::FeatureCompatibility::Version::kVersion50);
+    ServerGlobalParams::FeatureCompatibility::Version::kFullyDowngradedTo50);
 
 REGISTER_DOCUMENT_SOURCE_WITH_MIN_VERSION(
     _internalSetWindowFields,
     LiteParsedDocumentSourceDefault::parse,
     DocumentSourceInternalSetWindowFields::createFromBson,
     AllowedWithApiStrict::kNeverInVersion1,
-    ServerGlobalParams::FeatureCompatibility::Version::kVersion50);
+    ServerGlobalParams::FeatureCompatibility::Version::kFullyDowngradedTo50);
 
 list<intrusive_ptr<DocumentSource>> document_source_set_window_fields::createFromBson(
     BSONElement elem, const intrusive_ptr<ExpressionContext>& expCtx) {
diff --git a/src/mongo/db/pipeline/expression.cpp b/src/mongo/db/pipeline/expression.cpp
index 064caf30224..4d0f4a29893 100644
--- a/src/mongo/db/pipeline/expression.cpp
+++ b/src/mongo/db/pipeline/expression.cpp
@@ -7328,11 +7328,12 @@ void ExpressionDateTrunc::_doAddDependencies(DepsTracker* deps) const {
 }
 
 /* -------------------------- ExpressionGetField ------------------------------ */
-REGISTER_EXPRESSION_WITH_MIN_VERSION(getField,
-                                     ExpressionGetField::parse,
-                                     AllowedWithApiStrict::kNeverInVersion1,
-                                     AllowedWithClientType::kAny,
-                                     ServerGlobalParams::FeatureCompatibility::Version::kVersion50);
+REGISTER_EXPRESSION_WITH_MIN_VERSION(
+    getField,
+    ExpressionGetField::parse,
+    AllowedWithApiStrict::kNeverInVersion1,
+    AllowedWithClientType::kAny,
+    ServerGlobalParams::FeatureCompatibility::Version::kFullyDowngradedTo50);
 
 intrusive_ptr<Expression> ExpressionGetField::parse(ExpressionContext* const expCtx,
                                                     BSONElement expr,
@@ -7439,18 +7440,20 @@ Value ExpressionGetField::serialize(const bool explain) const {
 }
 
 /* -------------------------- ExpressionSetField ------------------------------ */
-REGISTER_EXPRESSION_WITH_MIN_VERSION(setField,
-                                     ExpressionSetField::parse,
-                                     AllowedWithApiStrict::kNeverInVersion1,
-                                     AllowedWithClientType::kAny,
-                                     ServerGlobalParams::FeatureCompatibility::Version::kVersion50);
+REGISTER_EXPRESSION_WITH_MIN_VERSION(
+    setField,
+    ExpressionSetField::parse,
+    AllowedWithApiStrict::kNeverInVersion1,
+    AllowedWithClientType::kAny,
+    ServerGlobalParams::FeatureCompatibility::Version::kFullyDowngradedTo50);
 
 // $unsetField is syntactic sugar for $setField where value is set to $$REMOVE.
-REGISTER_EXPRESSION_WITH_MIN_VERSION(unsetField,
-                                     ExpressionSetField::parse,
-                                     AllowedWithApiStrict::kNeverInVersion1,
-                                     AllowedWithClientType::kAny,
-                                     ServerGlobalParams::FeatureCompatibility::Version::kVersion50);
+REGISTER_EXPRESSION_WITH_MIN_VERSION(
+    unsetField,
+    ExpressionSetField::parse,
+    AllowedWithApiStrict::kNeverInVersion1,
+    AllowedWithClientType::kAny,
+    ServerGlobalParams::FeatureCompatibility::Version::kFullyDowngradedTo50);
 
 intrusive_ptr<Expression> ExpressionSetField::parse(ExpressionContext* const expCtx,
                                                     BSONElement expr,
@@ -7581,11 +7584,12 @@ Value ExpressionTsSecond::evaluate(const Document& root, Variables* variables) c
     return Value(static_cast<long long>(operand.getTimestamp().getSecs()));
 }
 
-REGISTER_EXPRESSION_WITH_MIN_VERSION(tsSecond,
-                                     ExpressionTsSecond::parse,
-                                     AllowedWithApiStrict::kNeverInVersion1,
-                                     AllowedWithClientType::kAny,
-                                     ServerGlobalParams::FeatureCompatibility::Version::kVersion50);
+REGISTER_EXPRESSION_WITH_MIN_VERSION(
+    tsSecond,
+    ExpressionTsSecond::parse,
+    AllowedWithApiStrict::kNeverInVersion1,
+    AllowedWithClientType::kAny,
+    ServerGlobalParams::FeatureCompatibility::Version::kFullyDowngradedTo50);
 
 /* ------------------------- ExpressionTsIncrement ----------------------------- */
 
@@ -7604,11 +7608,12 @@ Value ExpressionTsIncrement::evaluate(const Document& root, Variables* variables
     return Value(static_cast<long long>(operand.getTimestamp().getInc()));
 }
 
-REGISTER_EXPRESSION_WITH_MIN_VERSION(tsIncrement,
-                                     ExpressionTsIncrement::parse,
-                                     AllowedWithApiStrict::kNeverInVersion1,
-                                     AllowedWithClientType::kAny,
-                                     ServerGlobalParams::FeatureCompatibility::Version::kVersion50);
+REGISTER_EXPRESSION_WITH_MIN_VERSION(
+    tsIncrement,
+    ExpressionTsIncrement::parse,
+    AllowedWithApiStrict::kNeverInVersion1,
+    AllowedWithClientType::kAny,
+    ServerGlobalParams::FeatureCompatibility::Version::kFullyDowngradedTo50);
 
 MONGO_INITIALIZER_GROUP(BeginExpressionRegistration, ("default"), ("EndExpressionRegistration"))
 MONGO_INITIALIZER_GROUP(EndExpressionRegistration, ("BeginExpressionRegistration"), ())
diff --git a/src/mongo/db/pipeline/field_path.cpp b/src/mongo/db/pipeline/field_path.cpp
index 00dc46b520c..6a78400d9cc 100644
--- a/src/mongo/db/pipeline/field_path.cpp
+++ b/src/mongo/db/pipeline/field_path.cpp
@@ -100,7 +100,7 @@ void FieldPath::uassertValidFieldName(StringData fieldName) {
     const auto dotsAndDollarsHint =
         serverGlobalParams.featureCompatibility.isVersionInitialized() &&
             serverGlobalParams.featureCompatibility.isGreaterThanOrEqualTo(
-                FeatureCompatibilityParams::Version::kVersion50)
+                FeatureCompatibilityParams::Version::kFullyDowngradedTo50)
         ? " Consider using $getField or $setField."
         : "";
 
diff --git a/src/mongo/db/s/create_collection_coordinator.cpp b/src/mongo/db/s/create_collection_coordinator.cpp
index 02f43ee953e..b030289ab67 100644
--- a/src/mongo/db/s/create_collection_coordinator.cpp
+++ b/src/mongo/db/s/create_collection_coordinator.cpp
@@ -679,7 +679,7 @@ void CreateCollectionCoordinator::_createPolicyAndChunks(OperationContext* opCtx
          *_collectionUUID,
          ShardingState::get(opCtx)->shardId(),
          ChunkEntryFormat::getForVersionCallerGuaranteesFCVStability(
-             ServerGlobalParams::FeatureCompatibility::Version::kVersion50)});
+             ServerGlobalParams::FeatureCompatibility::Version::kFullyDowngradedTo50)});
 
     // There must be at least one chunk.
     invariant(!_initialChunks.chunks.empty());
diff --git a/src/mongo/db/s/resharding/resharding_coordinator_service.cpp b/src/mongo/db/s/resharding/resharding_coordinator_service.cpp
index 743815a3633..2c81cf671c6 100644
--- a/src/mongo/db/s/resharding/resharding_coordinator_service.cpp
+++ b/src/mongo/db/s/resharding/resharding_coordinator_service.cpp
@@ -892,7 +892,7 @@ ReshardingCoordinatorExternalStateImpl::calculateParticipantShardsAndChunks(
             coordinatorDoc.getReshardingUUID(),
             *donorShardIds.begin(),
             ChunkEntryFormat::getForVersionCallerGuaranteesFCVStability(
-                ServerGlobalParams::FeatureCompatibility::Version::kVersion50)};
+                ServerGlobalParams::FeatureCompatibility::Version::kFullyDowngradedTo50)};
         auto splitResult = initialSplitter.createFirstChunks(opCtx, shardKey, splitParams);
         initialChunks = std::move(splitResult.chunks);
 
diff --git a/src/mongo/db/s/shard_collection_legacy.cpp b/src/mongo/db/s/shard_collection_legacy.cpp
index 2730d7c3ee2..028df745a3d 100644
--- a/src/mongo/db/s/shard_collection_legacy.cpp
+++ b/src/mongo/db/s/shard_collection_legacy.cpp
@@ -562,7 +562,7 @@ CreateCollectionResponse shardCollection(OperationContext* opCtx,
                 serverGlobalParams.featureCompatibility.getVersion() == FCVersion::kVersion49) {
                 checkForExistingChunks(opCtx, nss, boost::none);
             } else if (serverGlobalParams.featureCompatibility.getVersion() ==
-                       FCVersion::kVersion50) {
+                       FCVersion::kFullyDowngradedTo50) {
                 if (auto optUUID = sharding_ddl_util::getCollectionUUID(opCtx, nss))
                     checkForExistingChunks(opCtx, nss, optUUID);
             } else {
diff --git a/src/mongo/db/s/sharding_ddl_50_upgrade_downgrade.cpp b/src/mongo/db/s/sharding_ddl_50_upgrade_downgrade.cpp
index 8e3a68eed6b..6f8fe9e4247 100644
--- a/src/mongo/db/s/sharding_ddl_50_upgrade_downgrade.cpp
+++ b/src/mongo/db/s/sharding_ddl_50_upgrade_downgrade.cpp
@@ -40,7 +40,10 @@ DatabaseEntryFormat::Format DatabaseEntryFormat::get(const FixedFCVRegion& fcvRe
     switch (fcvRegion->getVersion()) {
         case FCVersion::kUpgradingFrom44To50:
         case FCVersion::kUpgradingFrom49To50:
-        case FCVersion::kVersion50:
+        case FCVersion::kUpgradingFrom50To51:
+        case FCVersion::kDowngradingFrom51To50:
+        case FCVersion::kFullyDowngradedTo50:
+        case FCVersion::kVersion51:
             return feature_flags::gShardingFullDDLSupportTimestampedVersion.isEnabledAndIgnoreFCV()
                 ? Format::kUUIDandTimestamp
                 : Format::kUUIDOnly;
@@ -61,7 +64,10 @@ ChunkEntryFormat::Format ChunkEntryFormat::getForVersionCallerGuaranteesFCVStabi
             return feature_flags::gShardingFullDDLSupportTimestampedVersion.isEnabledAndIgnoreFCV()
                 ? Format::kNamespaceAndUUIDWithTimestamps
                 : Format::kNamespaceOnlyNoTimestamps;
-        case FCVersion::kVersion50:
+        case FCVersion::kFullyDowngradedTo50:
+        case FCVersion::kVersion51:
+        case FCVersion::kUpgradingFrom50To51:
+        case FCVersion::kDowngradingFrom51To50:
             return feature_flags::gShardingFullDDLSupportTimestampedVersion.isEnabledAndIgnoreFCV()
                 ? Format::kUUIDOnlyWithTimestamps
                 : Format::kNamespaceOnlyNoTimestamps;
diff --git a/src/mongo/db/s/sharding_ddl_util.cpp b/src/mongo/db/s/sharding_ddl_util.cpp
index 107bd3df64b..85682a5925c 100644
--- a/src/mongo/db/s/sharding_ddl_util.cpp
+++ b/src/mongo/db/s/sharding_ddl_util.cpp
@@ -313,7 +313,7 @@ void shardedRenameMetadata(OperationContext* opCtx,
         // Only bump the epoch if the whole cluster is in FCV 5.0, so chunks do not contain epochs.
         FixedFCVRegion fixedFCVRegion(opCtx);
         if (serverGlobalParams.featureCompatibility.isGreaterThanOrEqualTo(
-                ServerGlobalParams::FeatureCompatibility::Version::kVersion50)) {
+                ServerGlobalParams::FeatureCompatibility::Version::kFullyDowngradedTo50)) {
             fromCollType.setEpoch(OID::gen());
         }
     }
diff --git a/src/mongo/db/server_options.h b/src/mongo/db/server_options.h
index 7a5e3cd226f..c8aa3037866 100644
--- a/src/mongo/db/server_options.h
+++ b/src/mongo/db/server_options.h
@@ -165,64 +165,58 @@ struct ServerGlobalParams {
          * (Y, Y, X): Only Y features are available and new storage engine entries use the
          *            Y format, but existing entries may have either the Y or X format
          *
-         * kUnsetDefault44Behavior
+         * kUnsetDefault{LTS}Behavior
          * (Unset, 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 (kUnsetDefault44Behavior).
+         *                        will return the default (kUnsetDefault{LTS}Behavior).
          *
          */
         enum class Version {
             // The order of these enums matter: sort by (version, targetVersion, previousVersion).
             kInvalid,
-            kUnsetDefault44Behavior,
-            kFullyDowngradedTo44,    // { version: 4.4 }
-            kDowngradingFrom47To44,  // { version: 4.4, targetVersion: 4.4, previousVersion: 4.7 }
-            kDowngradingFrom48To44,  // { version: 4.4, targetVersion: 4.4, previousVersion: 4.8 }
-            kDowngradingFrom49To44,  // { version: 4.4, targetVersion: 4.4, previousVersion: 4.9 }
-            kDowngradingFrom50To44,  // { version: 4.4, targetVersion: 4.4, previousVersion: 5.0 }
-            kUpgradingFrom44To47,    // { version: 4.4, targetVersion: 4.7 }
-            kUpgradingFrom44To48,    // { version: 4.4, targetVersion: 4.8 }
-            kUpgradingFrom44To49,    // { version: 4.4, targetVersion: 4.9 }
-            kUpgradingFrom44To50,    // { version: 4.4, targetVersion: 5.0 }
-            kVersion47,              // { version: 4.7 }
-            kDowngradingFrom48To47,  // { version: 4.7, targetVersion: 4.7, previousVersion: 4.8 }
-            kUpgradingFrom47To48,    // { version: 4.7, targetVersion: 4.8 }
-            kVersion48,              // { version: 4.8 }
-            kDowngradingFrom49To48,  // { version: 4.8, targetVersion: 4.8, previousVersion: 4.9 }
-            kUpgradingFrom48To49,    // { version: 4.8, targetVersion: 4.9 }
-            kVersion49,              // { version: 4.9 }
-            kDowngradingFrom50To49,  // { version: 4.9, targetVersion: 4.9, previousVersion: 5.0 }
-            kUpgradingFrom49To50,    // { version: 4.9, targetVersion: 5.0 }
-            kVersion50,              // { version: 5.0 }
+            kVersion44,              // To be removed once old feature flags are deleted
+            kVersion47,              // To be removed once old feature flags are deleted
+            kVersion48,              // To be removed once old feature flags are deleted
+            kVersion49,              // To be removed once old feature flags are deleted
+            kFullyDowngradedTo44,    // To be removed once old feature flags are deleted
+            kDowngradingFrom50To49,  // To be removed once old feature flags are deleted
+            kDowngradingFrom50To44,  // To be removed once old feature flags are deleted
+            kUpgradingFrom44To50,    // To be removed once old feature flags are deleted
+            kUpgradingFrom44To47,    // To be removed once old feature flags are deleted
+            kUpgradingFrom49To50,    // To be removed once old feature flags are deleted
+            kUnsetDefault50Behavior,
+            kFullyDowngradedTo50,    // { version: 5.0 }
+            kDowngradingFrom51To50,  // { version: 5.0, targetVersion: 5.0, previousVersion: 5.1 }
+            kUpgradingFrom50To51,    // { version: 5.0, targetVersion: 5.1 }
+            kVersion51,              // { version: 5.1 }
         };
 
         // These constants should only be used for generic FCV references. Generic references are
         // FCV references that are expected to exist across LTS binary versions.
-        static constexpr Version kLatest = Version::kVersion50;
-        static constexpr Version kLastContinuous = Version::kVersion49;
-        static constexpr Version kLastLTS = Version::kFullyDowngradedTo44;
+        static constexpr Version kLatest = Version::kVersion51;
+        static constexpr Version kLastContinuous = Version::kFullyDowngradedTo50;
+        static constexpr Version kLastLTS = Version::kFullyDowngradedTo50;
 
         // These constants should only be used for generic FCV references. Generic references are
         // FCV references that are expected to exist across LTS binary versions.
         // NOTE: DO NOT USE THEM FOR REGULAR FCV CHECKS.
-        static constexpr Version kUpgradingFromLastLTSToLatest = Version::kUpgradingFrom44To50;
+        static constexpr Version kUpgradingFromLastLTSToLatest = Version::kUpgradingFrom50To51;
         static constexpr Version kUpgradingFromLastContinuousToLatest =
-            Version::kUpgradingFrom49To50;
-        static constexpr Version kDowngradingFromLatestToLastLTS = Version::kDowngradingFrom50To44;
+            Version::kUpgradingFrom50To51;
+        static constexpr Version kDowngradingFromLatestToLastLTS = Version::kDowngradingFrom51To50;
         static constexpr Version kDowngradingFromLatestToLastContinuous =
-            Version::kDowngradingFrom50To49;
+            Version::kDowngradingFrom51To50;
         // kUpgradingFromLastLTSToLastContinuous is only ever set to a valid FCV when
         // kLastLTS and kLastContinuous are not equal. Otherwise, this value should be set to
         // kInvalid.
-        static constexpr Version kUpgradingFromLastLTSToLastContinuous =
-            Version::kUpgradingFrom44To49;
+        static constexpr Version kUpgradingFromLastLTSToLastContinuous = Version::kInvalid;
 
         /**
          * 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::kUnsetDefault44Behavior;
+            return _version.load() != Version::kUnsetDefault50Behavior;
         }
 
         /**
@@ -275,7 +269,7 @@ struct ServerGlobalParams {
         }
 
         void reset() {
-            _version.store(Version::kUnsetDefault44Behavior);
+            _version.store(Version::kUnsetDefault50Behavior);
         }
 
         void setVersion(Version version) {
@@ -283,7 +277,7 @@ struct ServerGlobalParams {
         }
 
     private:
-        AtomicWord<Version> _version{Version::kUnsetDefault44Behavior};
+        AtomicWord<Version> _version{Version::kUnsetDefault50Behavior};
 
     } mutableFeatureCompatibility;
 
diff --git a/src/mongo/db/update/pipeline_executor.cpp b/src/mongo/db/update/pipeline_executor.cpp
index 3bd5ec062fe..c2e77fb1e90 100644
--- a/src/mongo/db/update/pipeline_executor.cpp
+++ b/src/mongo/db/update/pipeline_executor.cpp
@@ -104,7 +104,7 @@ UpdateExecutor::ApplyResult PipelineExecutor::applyUpdate(ApplyParams applyParam
     bool allowTopLevelDollarPrefixedFields =
         serverGlobalParams.featureCompatibility.isVersionInitialized() &&
         serverGlobalParams.featureCompatibility.isGreaterThanOrEqualTo(
-            FeatureCompatibilityParams::Version::kVersion50);
+            FeatureCompatibilityParams::Version::kFullyDowngradedTo50);
     auto ret = ObjectReplaceExecutor::applyReplacementUpdate(
         applyParams, transformedDoc, transformedDocHasIdField, allowTopLevelDollarPrefixedFields);
 
diff --git a/src/mongo/db/update/storage_validation.cpp b/src/mongo/db/update/storage_validation.cpp
index 12f7559fefd..444637470f3 100644
--- a/src/mongo/db/update/storage_validation.cpp
+++ b/src/mongo/db/update/storage_validation.cpp
@@ -115,7 +115,7 @@ void validateDollarPrefixElement(mutablebson::ConstElement elem) {
         const auto replaceWithHint =
             serverGlobalParams.featureCompatibility.isVersionInitialized() &&
                 serverGlobalParams.featureCompatibility.isGreaterThanOrEqualTo(
-                    FeatureCompatibilityParams::Version::kVersion50)
+                    FeatureCompatibilityParams::Version::kFullyDowngradedTo50)
             ? "' is not allowed in the context of an update's replacement document. Consider using "
               "an aggregation pipeline with $replaceWith."
             : "' is not valid for storage.";
@@ -140,7 +140,7 @@ Status storageValidIdField(const mongo::BSONElement& element) {
             if (!status.isOK() && status.code() == ErrorCodes::DollarPrefixedFieldName &&
                 serverGlobalParams.featureCompatibility.isVersionInitialized() &&
                 serverGlobalParams.featureCompatibility.isGreaterThanOrEqualTo(
-                    FeatureCompatibilityParams::Version::kVersion50)) {
+                    FeatureCompatibilityParams::Version::kFullyDowngradedTo50)) {
                 return Status(status.code(),
                               str::stream() << "_id fields may not contain '$'-prefixed fields: "
                                             << status.reason());
@@ -203,7 +203,7 @@ void storageValid(mutablebson::ConstElement elem,
     const bool dotsAndDollarsFeatureEnabled =
         serverGlobalParams.featureCompatibility.isVersionInitialized() &&
         serverGlobalParams.featureCompatibility.isGreaterThanOrEqualTo(
-            FeatureCompatibilityParams::Version::kVersion50);
+            FeatureCompatibilityParams::Version::kFullyDowngradedTo50);
     const bool checkFields = !dotsAndDollarsFeatureEnabled || checkTopLevelFields;
 
     auto fieldName = elem.getFieldName();
diff --git a/src/mongo/db/wire_version.h b/src/mongo/db/wire_version.h
index 45712e1c298..9d1c72d20f3 100644
--- a/src/mongo/db/wire_version.h
+++ b/src/mongo/db/wire_version.h
@@ -95,16 +95,19 @@ enum WireVersion {
     // Supports features available from 5.0 and onwards.
     WIRE_VERSION_50 = 13,
 
+    // Supports features available from 5.1 and onwards.
+    WIRE_VERSION_51 = 14,
+
     // Set this to the highest value in this enum - it will be the default maxWireVersion for
     // the WireSpec values.
-    LATEST_WIRE_VERSION = WIRE_VERSION_50,
+    LATEST_WIRE_VERSION = WIRE_VERSION_51,
 
     // Set this to LATEST_WIRE_VERSION - 1.
     LAST_CONT_WIRE_VERSION = LATEST_WIRE_VERSION - 1,
 
     // Set this to the wire version of the previous LTS version. We expect to update this after
     // each LTS release.
-    LAST_LTS_WIRE_VERSION = RESUMABLE_INITIAL_SYNC,
+    LAST_LTS_WIRE_VERSION = WIRE_VERSION_50,
 };
 
 /**