Interesting,
Called a Verizon store and the guy there said anytime I change the phone (unless its to an identical model), I would lose grandfathered status on the data plan. I couldn't believe it and asked him then what would happen if I get a new phone and in 2 years I want to upgrade to a newer model --- he said I would lose grandfathered status.
So I hung up and called another Verizon store and the guy there said that as long as you stay on a smartphone the unlimited plan should be grandfathered forever. It's only when you downgrade to a non-smartphone that you could lose grandfathered status (but even then he said there would probably be a grace period).
Frustrating how no one knows for certain.