iPhone 4s owner going to 5s today. How can I keep my unlimited data on Verizon ?

I've had the unlimited data plan grandfathered for so many years now and would love to keep it when going to the 5s and lte.

Is it possible to keep my plan ? If so how ? Better to buy online ? In store ?

Do I have to pay full retail price for phone or foul I still take advantage of my upgrade ?

I know when I was in the apple store last year considering the iPhone 5 the Verizon rep kept saying I had to change the plan to get lte working... Somehow I don't believe him

Thanks !