Just to confirm, are you using IP Range scans for your targets? It does make a difference, as different scan types are stored in different tables. I have successfully update IP Range scans at the database level, mostly to make mass schedule changes or re-assign a group of ranges to a different scanner.
Changing credentials is a different ballgame, since you are working with correlation between multiple tables. It would be helpful to figure out, though; I find assigning credentials to multiple targets is very tedious.