I have a very basic understanding of legally binding agreements, but I think these would be the main scenarios we'll encounter assuming that Verizon goes through with this:
A.) When the change takes effect, upgrading any device in your plan will renew your contract and lock you into the new terms. You'll be warned about this and have a chance to agree or disagree, however.
B.) If under contract on 3G Unlimited, and you purchase a 4G phone for full retail after the change takes effect, Verizon will force you into tiered service regardless if you haven't renewed the contract. The defense will be that the level of service you're requesting (4G is considered a separate service from 3G, even though it's still data) is structured in tiers which are your only options. Grandfathering in was considered a courtesy to customers.
C.) You never upgrade your phone with subsidies and your contract has expired. You're now on a month-to-month basis. If Verizon goes full evil, they will cancel your service (no contract, so they aren't required to provide service until you sign back on) until you sign a contract. This new contract will have your grandfathered data eliminated.
Any legal aces out there, feel free to correct me if I'm wrong.