I have the 3G (got it on launch date) and want to upgrade to the 3GS. I have already checked all the threads and stuff and I am eligible for the "early upgrade of $399 - $499. My question is, I am on a family plan with my mom and dad. Both of my parents have old normal phones and don't care about upgrading at all. I checked and they are both eligible to upgrade to the 3GS at $199 - $299. Is it possible to use one of their upgrades and just swap our SIM cards after we leave the AT&T store? Obviously I want to keep my number and have everything else the same, but does anyone with experience know if I can utilize the upgrade that both of my parents have?