convert 3g to 4g mifi without losing unlimited Verizon data?

I also have a grandfathered unlimited data plan and I believe you can keep the old plan if you purchase the equipment at full price.

As others have said, don't trust what is verbally said to you by customer reps. Go into a Verizon store and confirm what was told to you by phone. It won't hurt if you also record the conversation with the store clerk in case Verizon does not honor what was said to you at a later time. Good luck