核心数据迁移:改变类型的持久性存储

I really need help to change Core Data structure. I have built my app already with existing database but now I would like to use Seam cloudKit sync and I have to change my code but if I do that my app will crash! (delete and build the app again is not an option.. as I mention it has already its data structure)I really need help to change Core Data structur