I am thinking about buying a used iPhone 4S since I am not eligible for an upgrade yet. Will I be able to go get a SIM card for it at AT&T and will I have to change my current plan? Thanks for any help.
You should able to go into a corporate store and they will give you a micro SIM if you bring your own phone. As for the plan, you will have to get an iPhone data plan, but your voice and text plans won't change.