I assume you are referring to the Boost Mobile $50 plan? You didn't mention your carrier. I've read here that they will allow you to covert to CDMA and keep the Monthly Unlimited level you currently pay, i.e. $35-50 if you are on the Shrinkage plan. As far as avoiding Blackberry fees, they don't technically "allow" what most of us Boostberry 8350i users do now... it's just a software workaround and they don't have a way to restrict it. So chances are you'd be paying the BB fee with a new 8530 model phone.
But you could call them and ask, say you don't want any of the specific BB features (like e-mail push) and just want to use the phone/text features. Maybe they'll have a way to program that and charge you accordingly.
P.S. The 85xx forum section might be a better place to ask this if you haven't already.