AT&T iPhone4 question, dealing with data plan.?

My boyfriend wants to get me a iphone4 for my birthday, (my iphone 3gs is slowly going to pot) My upgrade isn't till april, and its silly paying 200 extra dollars. SO, he wants to use his upgrade to get me the ip4 now, and we need to know if you can automatically stop the data plan right after you get the phone in order to stop the charges to his bill. He is using my blackjack 2 and it works perfectly. I have a crack in my LED screen and the woman at AT&T said if my screen goes completely out ill lose all of my information. Please someone let me know if this is possible, with some backing behind your answer!