If you need to release a new version of your app with updates to the model then you must create a new model version and make changes to the model ONLY in the new model version.  You must also keep the old model version in the bundle because Core Data needs the old version to open existing files prior to migrating them to the new model.

Lightweight migration should work find but be aware that it can take some time to complete and also be aware that synchronisation will not work between apps using different model versions.  So make sure your users upgrade all their devices at the same time and get them to avoid entering data until the upgrade has been completed.

Always a good idea to make a backup before any such upgrade, however you can’t rely on Core Data to make backups once the new model version is included in the App.