Unless you have agreed differently with the artist, they retain the rights to the finished image. Regardless of the characters depicted therein, unless you own a trademark or registered copyright.
The time frame depends, I think you can always ask an artist about changes. Asking doesn't hurt, provided the artist is a normal person. If the change is small and the artist is nice the odds are good that they'll help you out. Otherwise they might ask for compensation for the additional work.
no subject
The time frame depends, I think you can always ask an artist about changes. Asking doesn't hurt, provided the artist is a normal person. If the change is small and the artist is nice the odds are good that they'll help you out. Otherwise they might ask for compensation for the additional work.