Any chances that anyone who already has an unlimited plan will be "grandfathered" in to keep the plan? I have never actually looked, but as far as I know I have never used that much. Mostly just basic browsing, and looking on the Market. Now at work sometimes I stream Pandora so I might be using more than I think, but I still don't think it is that much.
Verizon frequently "grandfathers" plans on a selective basis. (I have the same family plan I've had for five years.) However, they are certainly not required to do so and they use various incentives to "encourage" users to adopt new plans. For example, some phones are not available if one wishes to keep a particular plan.
In this case, it's very likely there will no "grandfathering" if you want a 4G phone. And if that doesn't limit the number of "grandchildren" seeking to keep unlimited data plans, there would be nothing to prevent VZW from excluding high bandwidth users from any grandfathering.
The only real guarantee here is that VZW won't change the terms of your current contract. (That would be illegal.) And I suspect that they would not try to force you off the network if you continue to pay on a month-to-month basis once your contract ends even though that is a bit "grayer" area.