Hi, So here's my current situation with AT&T. I currently have an iPhone 3G that I bought last summer and want to upgrade to the 3GS. I'm on a family plan, and although my line isn't upgrade eligible, the other line is. What I want to do is sell my 3G and buy the 3GS on the other line, and switch numbers. I assume I can do this, as its what I did last year with my 3G when my line wasn't eligible but another line was. So here are my two questions: 1) When I sell my iPhone, is there anything I need to do in terms of AT&T, or can the other guy just insert his sim card? 2) Will I be charged twice for a data plan when I buy the 3GS? Thanks!