I want to upgrade to a 3GS but Im not eligible for an upgrade because I got the 3G on launch day. However, one of the lines on my family plan is eligible for an upgrade (my step dads line). My question is this: If I go and buy a 3GS using the eligible line, can I then take the sim out of my current 3G and put it in my new 3GS so that I can keep my current number instead of having to use my stepdads number? Any help is MUCH appreciated!!