I can say that although those clauses 'may' exist, I know for certain that ATT has grandfathered users for the unlimited plan as I have a friend who is with ATT who had this exact issue occur. He called them, and stated he didn't agree to the new pricing but would remain under the old, which was agreed to and ATT switched him back.
Also, your pricing even after contract rarely changes, unless you renew for a phone upgrade, as I have had phone pricing on prior models that the plan I had hadn't actually been available for years with Verizon, yet I still had it.
Anyway, that is my experience, Your mileage may vary of course.