diff --git a/mmv1/products/secretmanager/Secret.yaml b/mmv1/products/secretmanager/Secret.yaml
index c2e8a0832e77..1fe39265855f 100644
--- a/mmv1/products/secretmanager/Secret.yaml
+++ b/mmv1/products/secretmanager/Secret.yaml
@@ -49,6 +49,8 @@ examples:
 import_format: ['projects/{{project}}/secrets/{{secret_id}}']
 custom_code: !ruby/object:Provider::Terraform::CustomCode
   pre_update: templates/terraform/pre_update/secret_manager_secret.go.erb
+  constants: templates/terraform/constants/secret_manager_secret.go
+custom_diff: ['secretManagerSecretAutoCustomizeDiff']
 parameters:
   - !ruby/object:Api::Type::String
     name: secretId
@@ -125,7 +127,8 @@ properties:
     properties:
       - !ruby/object:Api::Type::Boolean
         name: automatic
-        immutable: true
+        # Immutability is handled by the custom diff function until "automatic" is removed
+        # immutable: true
         exactly_one_of:
           - replication.0.automatic
           - replication.0.user_managed
@@ -137,7 +140,8 @@ properties:
       - !ruby/object:Api::Type::NestedObject
         name: auto
         api_name: automatic
-        immutable: true
+        # Immutability is handled by the custom diff function until "automatic" is removed
+        # immutable: true
         exactly_one_of:
           - replication.0.automatic
           - replication.0.user_managed
diff --git a/mmv1/templates/terraform/constants/secret_manager_secret.go b/mmv1/templates/terraform/constants/secret_manager_secret.go
new file mode 100644
index 000000000000..13db855f5e7e
--- /dev/null
+++ b/mmv1/templates/terraform/constants/secret_manager_secret.go
@@ -0,0 +1,25 @@
+// Prevent ForceNew when upgrading replication.automatic -> replication.auto
+func secretManagerSecretAutoCustomizeDiff(_ context.Context, diff *schema.ResourceDiff, meta interface{}) error {
+	oAutomatic, nAutomatic := diff.GetChange("replication.0.automatic")
+	_, nAuto := diff.GetChange("replication.0.auto")
+	autoLen := len(nAuto.([]interface{}))
+
+	// Do not ForceNew if we are removing "automatic" while adding "auto"
+	if oAutomatic == true && nAutomatic == false && autoLen > 0 {
+		return nil
+	}
+
+	if diff.HasChange("replication.0.automatic") {
+		if err := diff.ForceNew("replication.0.automatic"); err != nil {
+			return err
+		}
+	}
+
+	if diff.HasChange("replication.0.auto") {
+		if err := diff.ForceNew("replication.0.auto"); err != nil {
+			return err
+		}
+	}
+
+	return nil
+}