If you don't buy a discounted phone (do nothing, your contract will continue with unlimited data until you cancel. In December, you will be considered a "month to month" account, no change in your plan.
If, at any time, you purchase a discounted phone from Verizon or one of it's agents (Best Buy, Costco, Amazon, etc), you will be "forced" into a tiered data plan or Share Everything account (depending n where and how the purchase is made).
If you purchase a new phone from a private party, or continue to use your current phone, your unlimited data will remain. Keep in mind that it is an added feature, and can be changed at any time by Verizon. So far they are allowing the unlimited to remain as long as you do not sign a new contract and continue in a month to month status.