Yes. Pay full price and you will keep unlimited data now on LTE even if you come from a 3G iPhone 4/4S. I can attest to this (and also kept a cheaper legacy voice plan). Try another sales person if they're telling you otherwise.
Whether this will always last, I can't say. But they'd be smart to continue permitting it. I'd switch to T-Mobile otherwise.
as cramer would say - BOOOOYAHHHH
and here i thought i couldn't keep unlimited data if i switched to a LTE phone. man verizon has it coming...i already use 5GB-6GB each month on regular 3G