existing references aren’t cleaned up in the database, since on synchronization Mendix cannot decide which to keep and which to delete.
My advise would be, clean up the data before changing the association type. Or even better add a new 1-1 association, copy the assciation you want to maintain and then delete the 1-* association
Having them not cleaned up can also cause unique constraint violations. If you use DB based unique constraint validation (Default on Mendix 7 and higher), you app will not start up if there is such a violation.
This is what the modeler does when adding an association between “Entity” and “Entity_2”. It’s adding a constraint.
I reproduced it by first having one-many. I added three objects of entity that all had an association to a single object of entity_2.
When now changing the association to one-one, the app is not starting up because of a unique constraint violation.
uniqueness constraint are related to associations.