Let the hive mind of Engadget get that for you.
"I just switched to Sprint from Verizon about three months ago for the Pre. Then I went for the Hero about a week ago. Now, I miss my hardware keyboard and am thinking about switching to the Moment. I am still able to switch back to Verizon if I want and get the Droid when it arrives. Should I just trade up to the Moment when it comes out, see if I like it, and if not switch to the Droid? Or something else entirely? Help!"
Well if you think about it, it would apply to anyone, since even people with a text messaging plan have to pay the new higher rate if they were to go over. The only point of contention is if you have an unlimited text plan, but generally when something like this happens, the call centers know a change was made and people can get out of the contract, so I doubt they would actually fight with you over the fact that you had unlimited messaging.