In short you don't
Unless you have a good bit of legal know-how their priority is not to pander to you.
You could always call and throw around some big words until you scare the phone girl into passing you up the chain of command but with 70million other people who use their service anyway i doubt AT&T will be to unhappy to screw over one converter.
They have no obligation nore do they have any real reason. I say just go ahead cancel out and sell your phone. After that just sign up with cingular for the moderate fee.
Playing these companies for monney is a great way to have them just decide to put you on their sh** list and thats not where you want to be unless your donald trump and can afford to rent out their executive boards as your own personal anal concubines.
Good luck though if you can think of a way