I have a family plan. My phone is the iPhone 3G and the other phone is some cheapo flip phone which is eligible for an upgrade and belongs to my niece. If I order the 3Gs as an upgrade for the cheapo phone, can i put my iPhone 3G SIM in it and it (3Gs) work with my current phone number.. then put the other SIM card in my old 3G iPhone and give that to my niece? My goal is to be able to get the 3Gs and keep my current phone number. And give my 3G to my niece and she keep her current phone number.