I am currently on an AT&T family plan with 3 other phones. My line isn't eligible for an upgrade until 2010 but I still want the full subsidized price for the iPhone 3GS. One other line in my plan is eligible this month. I was wonderi ng if I could just upgrade that line to the iPhone 3GS and then just call AT&T about a week later to switch the data plan to my line and use the iPhone myself?