As long as you continue with your unlimited data plan, you will be grandfathered in. Just don't let it get removed, it's difficult to get re-added.
As for your prepaid question, they should be able to add it to whichever prepaid plan they choose, but with an additional $30/month charge for the smartphone data. Most, if not all, markets should be on the new unlimited plans (formerly known as "unleashed") but the included unlimited data only applies to basic phones.
Dan K. | Connectivity Business Group Any opinions expressed in this post are those of the author and do not represent Best Buy Co., Inc.