You'll need to change your SIM card, which might draw a red flag now since all of the Iphone SIM cards are MICRO SIMS now. Your best bet at trying this is buying your phones and not letting them take a peep at your account in the store. When you get home just call into customer service and tell them you have a new sim card number for them. NOT IMEI! On Verizon I don't know what you would have to do unless the new Iphone is 4g capable. (No IMEI or Sim Card in non 4g Verizon Phones)
My suggestions, stay with Android. Technology in the real world is going to be following android based devices as the future unfolds, not Apples IOS platform. I'm not an Apple hater, as I have a few thousand dollars worth of their products, but when it comes to the phones Androids win IMO.
Wifi? 4g is faster than your home internet. Notice how since the 4g has come out Verizon even dropped their plan down to 2gb of data. The phone companies are thinking into the future already. If you have unlimited data, do whatever you can to keep it, even if that means not upgrading to that IPhone.