Thanks Darkpaw would you have a link to a tutorial for a proper data migration ? Because I got your idea but have difficulties to know what is exactly meant by 'new version of the model' (does it mean a new data model within the app with a different name?), 'mapping model' and I have no idea how to tell Core Data to do such operation. Sorry.

Thanks Claude, as mentionned in the post below, problem is on deleting old data. Once data is saved in backup file, I used a batchDeleteRequest to flush all data on the entity where I replaced the attribute by a relationship. But I got runtime exception on the contentView, where that entity is listed (the code of the batchDeleteRequest being in a child view of that contentView). Honnestly I did not try further. I found it was easier to delete and reinstall the app.

With a short example how to use that would be so nice for the newbies like me. Thanks! I need the stored date to be nil until it is defined.