Some normal users like myself also have the ability to change best answers. I'm not sure what the credentials are that gave me these godlike powers. If you give a link to the thread in question and who you want to give the credit to, I can change it for you.
I don't think you can, because people get credit for best answer which goes towards badges and such. If they let you change the best answer, they'd have to let people lose badges, which would be a moderator nightmare.
If they let two people get credit for the same question, then people would complain if they weren't also given credit all the time.