How Do I Buy Iphone 3g From Friend And Get It The Plan Switched To My Account?

I am not eligible for an upgrade until January but I want the iphone 3g now. My friend has the iphone 3g and wants to sell it to me. If I go to cingular will they switch the data plan to my accound even though I didnt buy the iphone from them? Thanks ahead of time.  Read more...