I recently purchased an At&t iPhone 4S from a friend. I currently have a non-smartphone from At&t and I just upgraded my plan to unlimited data (for non-smartphones). I know that I will have to cut down my sim card, but other than that, what do I have to do to get my new iPhone working with this plan. Do I have to unlock it? Can I just switch the sims and be set? If neither of those, what about grandfathering in an unlimited data plan (I know people who have unlimited data on their iPhones this way). I want to keep unlimited data on my iPhone. Thanks!