My Mom and I both recently upgraded to the iPhone 4S from the iPhone 4. They'll be here tomorrow. Thing is, we didn't upgrade on our own lines. I have my own, separate ATT account. My mom has a family plan on her account with 4 active lines. Two of those lines had an available upgrade, however ours did not. I've upgraded on her account, but I intend to use the phone on my own. She upgraded on a different line on her account, but intends to use the iPhone 4S with her number. Logically, we should be able to pop out our SIM cards and stick them in the 4S, right? However, I heard otherwise. I heard, in passing, that this was much more difficult than I thought it would be. Has anyone else cross upgraded on ATT? How does it work? Am I in for a difficult process tomorrow?