HOW DO I BUY IPHONE 3G FROM FRIEND AND GET IT THE PLAN SWITCHED TO MY ACCOUNT?

I am not authorised for an ascent until Jan but I wish the iphone 3g now. My crony has the iphone 3g and wants to sell it to me. If I go to cingular will they switch the interpretation devise to my accound even yet I didnt buy the iphone from them? Thanks forward of time.