To options for what we could do if someone wants to change the rating scale after some ratings have been submitted:
1) Block changes in scale if there are already ratings using them (modedit).
2) Allow to delete existing / re-escale existing if the teacher wants to unlock the setting (based on some cap/s).

Eloy says: I think this can be achieved in a similar way than activity completion is doing right now, take a look to if as reference.