Legally if you don't change the plan, or haven't done anything that'll force you to change the plan, they cannot change the plan on their own, without your authorization.
And buying a phone in full price (non-subsidized), without a contract won't require you to change your plan, AT ALL.
I did, just deactivate the old phone and activate the iPhone 5, done. No need to call them at all as long as VZW knows this phone is a "replacement".
100% BS
They can eliminate the unlimited data plan at ANY time, without restriction.
You have NO contract with Verizon, even if you pay full price, that states the unlimited plan will continue indefinitely.
Now that 1000's of unlimited LTE iPhone users are abusing the system, it's a little different than when it was "only" 3G/+. At SOME point, they will force users off the plan. They may not be able to throttle, but it DOESN'T MATTER. They can simply eliminate the plan. Then you have to choose a tiered plan.
They have no obligation to continue the plan. None.
And before you tell me about how valuable your business is, realize they'd probably be willing to take that chance, seeing as how only Sprint (lol) has unlimited these days.
Face it, you paid full price to keep a feature that is month-to-month. Nothing more. Sorry.
(and no, not jealous... I'm on the wonderful AT&T throttle me at 5GB LTE "unlimited" plan after escaping VZ years ago...)