Here's the scoop straight from my verizon store rep, he's usually straight up with me and gives me the inside scoop.
According to him: If you have unlimited data you keep it, there is nothing they can do about that... UNTIL you upgrade your phone. Now, here's the tricky part: If you're upgrading from 3g to 3g your unlimited stays, if you're going from 3g to 4g and want to keep your unlimited that's when they're going to get you by saying "LTE Unlimited is no longer available."
So you can still keep your grandfathered unlimited plan, just not on 4g. I'm lucky enough that I slid in right before this all happened... I got my GNex about two months ago and I am still good to go on my truly unlimited plan (same with my mom, she has some other samsung 4g phone), and thats when that guy told me if I had waited until july to upgrade my phone I would lose the unlimited data.
So I guess I was one of the lucky last ones... i'm interested to see what they do for loyal (5+ year) customers who have had unlimited since the beginning and they try to kick them off, i'll bet its not going to be pretty.