Potentially this is the case. Although, I have never seen any customers who are mandated to upgrade their plan especially if they retain the same hardware, aside from two occasions where I had customers call about a 10+yr old Alltel plan.
Some people don't understand what grandfathered means. It doesn't mean you can keep the plan/feature forever and change other components of your plan/account, but it means you are allowed to keep the plan/feature barring those types of changes. Technically some hardware changes affect that as well. When you upgrade (get the discounted price toward a phone), you have to get a new contract. When you get a new contract, you need "current pricing." Which grandfathered features never are =\
So the ways around that are to leave that line's contract alone. Pay full price, use the upgrade from a different line, whatever takes to not extend the contract.