As long as you aren't changing anything to your plan (as in adding new lines, more minutes, texts, ect), you should be grandfathered with your previous plans. If they tell you otherwise, ask to speak with a manager because they probably are told to push for people to break away from unlimited plans.
Man, that stinks. I just changed my texts from 500 to pay-per-minute yesterday because I started using Google Voice. What will happen when LTE comes along in my area? Will my plan just stay on 3G, or will my unlimited data that's still there now just be taken away and substituted?