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

We downgraded out plan for the winter (at the reps recommendation and not the unlimited plan). We confirmed multiple times that it would be no problem returning to our original plan.

Never ever change your plan..unless you are sure you want that permanently, there is no such thing as changing and then going back
on occasion..under 30 days, aka 15 day grace period, you can revert
but after the grace period it takes a miracle of cajoling to revert back to your previous plan