fix: preserve api mongo_db_major_version on real upgrades

internal/service/advancedcluster/resource_compatiblity.go

@@ -94,7 +94,8 @@ func overrideAttributesWithPrevStateValue(modelIn, modelOut *TFModel) {
 		return
 	}
 	beforeVersion := conversion.NilForUnknown(modelIn.MongoDBMajorVersion, modelIn.MongoDBMajorVersion.ValueStringPointer())
-	if beforeVersion != nil && !modelIn.MongoDBMajorVersion.Equal(modelOut.MongoDBMajorVersion) {
+	afterVersion := conversion.NilForUnknown(modelOut.MongoDBMajorVersion, modelOut.MongoDBMajorVersion.ValueStringPointer())
+	if beforeVersion != nil && afterVersion != nil && shouldUsePreviousMongoDBMajorVersion(*beforeVersion, *afterVersion) {
 		modelOut.MongoDBMajorVersion = types.StringPointerValue(beforeVersion)
 	}
 	overrideMapStringWithPrevStateValue(&modelIn.Labels, &modelOut.Labels)
@@ -109,6 +110,10 @@ func overrideAttributesWithPrevStateValue(modelIn, modelOut *TFModel) {
 	modelOut.UseEffectiveFields = modelIn.UseEffectiveFields
 }
 
+func shouldUsePreviousMongoDBMajorVersion(beforeVersion, afterVersion string) bool {
+	return beforeVersion != afterVersion && FormatMongoDBMajorVersion(beforeVersion) == FormatMongoDBMajorVersion(afterVersion)
+}
+
 func overrideMapStringWithPrevStateValue(mapIn, mapOut *types.Map) {
 	if mapIn == nil || mapOut == nil || len(mapOut.Elements()) > 0 {
 		return

internal/service/advancedcluster/resource_compatiblity_test.go

@@ -0,0 +1,50 @@
+package advancedcluster
+
+import (
+	"testing"
+
+	"github.com/hashicorp/terraform-plugin-framework/types"
+)
+
+func TestOverrideAttributesWithPrevStateValueMongoDBMajorVersion(t *testing.T) {
+	testCases := []struct {
+		name     string
+		before   string
+		after    string
+		expected string
+	}{
+		{
+			name:     "keeps previous version when only formatting differs",
+			before:   "8",
+			after:    "8.0",
+			expected: "8",
+		},
+		{
+			name:     "uses API value when major version actually changed",
+			before:   "7.0",
+			after:    "8.0",
+			expected: "8.0",
+		},
+	}
+
+	for _, tc := range testCases {
+		t.Run(tc.name, func(t *testing.T) {
+			modelIn := &TFModel{
+				MongoDBMajorVersion: types.StringValue(tc.before),
+				Labels:              types.MapNull(types.StringType),
+				Tags:                types.MapNull(types.StringType),
+			}
+			modelOut := &TFModel{
+				MongoDBMajorVersion: types.StringValue(tc.after),
+				Labels:              types.MapNull(types.StringType),
+				Tags:                types.MapNull(types.StringType),
+			}
+
+			overrideAttributesWithPrevStateValue(modelIn, modelOut)
+
+			if got := modelOut.MongoDBMajorVersion.ValueString(); got != tc.expected {
+				t.Fatalf("unexpected mongo_db_major_version: got %q want %q", got, tc.expected)
+			}
+		})
+	}
+}