Usually they'll grandfather in unlimited plans that are in place before they kill them, though. I still have an "unlimited" data plan from Verizon, which, as I understand, will unfortunately not transfer over to an [EDIT: unlimited] LTE plan.
So, you should look into it, and if you're still planning to switch, you should probably try to do it before the iPhone comes out.
I have an unlimited data plan on Verizon, and was told last week it could transfer to LTE. (Didn't make the switch though; I opted for the Droid 3 and its physical keyboard.)
I don't know why you are getting downvoted. I've been involved in and also seen plenty of corporate cellphone deals where the carrier promises stuff (on paper, even) and then never delivers. "Billing mistakes" drag on and on, costing the customer huge bills they supposedly weren't going to get, etc.
On the consumer side, I've seen plenty of helpful customer service people do something slightly wrong and then the customer is locked out of their grandfathered plan or whatever else.
So yes, I agree. What they say and what is true don't always have anything to do with one another.
(One caveat to this is that sometimes only a few people know how to do something, and to everyone else it is "impossible." Sometimes this is poor training, and sometimes it is a rep who knows how to cheat the system a bit for a better customer experience.)
So, you should look into it, and if you're still planning to switch, you should probably try to do it before the iPhone comes out.