(1) Is it possible to just take the sim card out of an old iphone 4 and put it in an iphone 4S and use it? They are the same size correct? (2) Will At&t count that as an upgrade? Or will they not even really know about it? (3) Do iphone 3GS's and Iphone 4S's have different sized SIM cards? If you get a used iphone 4S can you take the sim out of a 3GS and put it in a 4S? Or do you need to contact AT&T for a new SIM card? And if you do that will they count that new sim card as an UPGRADE? Thanks for your help!!