When you need to change your Core Data Entities or structure, you have to add new model version before you send your App in AppStore. If you change just attributes or fetched templates, it’s not needed, you can do it without consequence. But if you want to add a new Entity, you have to migrate your persistent store to a new model version. You should also set mappings for the migration. In this tutorial, we add model versions, model mappings, and using Lightweight Migration.