I am going through a similar scenario and created a copy of my app to test the change and consequences it will have. What I found is that it did not adapt and change the related field to the new Key field. Instead, it disabled the relationship and the information that was being transferred over was no longer there.
In conclusion, I have to make this change so at this point with the research I have done and the inquiries I have made, it appears that I am going to have to recreate the relationships.
If anyone else can contribute to my findings... It would be appreciated as well!