It really depends where your jeweler is in the process. IF they have not started casting, then I would think your changes can be made. Though the bench jeweler may 'moan n' groan' - I wouldn't hesitate to at least ask. And don't feel badly, because as I said before, once cast, it's yours and there would be additional charges then. If you can catch it before hand, there should not be more $$ added to the current charge.
Good Luck.