I'm a AT&T customer since May 2008, on a 1400 family plan with 4 lines. My line is on the iPhone 2G plan (part of the family plan). If I want to upgrade one of the lines to iPhone 3G, how can I qualify for the $199/$299 price point? Can I upgrade to the 3G on my line, and switch the SIM later after the purchase/activation? Thank you.