I'd really advise against dropping from a 4G device to a 3G, if maintaining unlimited data is a serious concern. There was speculation that that 3G unlimited data = 4G unlimited data, but this turned out to be false recently when details on the tiered family shared data plans came to light.
If you absolutely must have an iPhone wait for the 5 which supposedly is going to be 4G capable. As far as buying a new phone goes, if you want to maintain that unlimited data you have to buy full phone prices. No upgrade discounts.
Good luck.
PS - for those of you dealing with getting new SIM cards, I HIGHLY recommend avoiding the retail store. Go onto Verizon's website, they will overnight SIM cards to you for free (might be 2 day, not sure) and you can activate the phone yourself. I don't trust in store reps, they always fudge with the accounts. Plus they make you sign off whenever they give you a new SIM in store, so I would highly recommend avoiding that at all costs.