You might want to call up Verizon, and, speak with someone who can give you a guarantee, and make sure to take down their employee ID. I have asked this question of every rep I've talked to this year. Some have said yes, some said no. Some even said they didn't know that Verizon was going to a tiered data plan. So, you might want to call up and ask to speak to a Supervisor, or, someone with some authority/knowledge on the matter.
I have a sneaking suspicion that if you sign a new contract they will try and switch you over. I'd like to believe, and, definitely hope that we will be grandfathered in, but, I just don't see it happening. With AT&T moving away from unlimited data, and, Verizon following, it seems like it will shortly be a thing of the past.
However, if you buy your new Bionic and don't sign a contract to do so, then you should be able to keep your unlimited data no matter what. And, if you use a lot of data as you say, it might be more cost effective in the long run, depending on how long you keep phones for, to just go down this path.