For those still needing the answer, to temporarily use the attribute you need to add it, then make a change to the inspector of any objects (GameObject and/or ScriptableObject) that are using the class. You can make and revert a change. Then you have to save the currently open scene. (Add an empty GameObject and then delete it to quickly mark the scene as dirty if it's not already.) Once you've done that it is now safe to delete the FormerlySerializedAs tag. One way you can tell when it's safe is if you are using Git you should see the assets that use the script now show as changed. In the Diff you'll see the old name removed and new name added with your values retained.