Neither Verizon nor any other network provider can't force you to switch - you signed a 2-year contract with them, and as part of that contract you got unlimited data plan. What they can do is:
1. When it's time to renew your contract, no longer give you an option of unlimited data. You can decide to go somewhere else, or sign a new contract.
2. Change your contract terms and switch you to tiered plan - that means though that you have a right to cancel the contract all together without any penalty (early termination fee), as there was a change to the original terms.
In either case, you have an option.
Polbit