The definition of "prepaid" is like defining "guaranteed" (especially at MR). We each have our interpretation. Even with qualifiers of no refunds, non-changeable, and no modifications; there is wiggle room. Plus, the property is able to make whatever concessions they choose. I have experienced being able to move the arrival dates (due to IROPS with flights), illness changing travel plans, problems with property or area prior to arrival (wild fires), and yes, rate changes.

So, in my view, if you want something different, ASK (hopefully nicely) and see what they say. They are not obligated to oblige. If they've collected the pre-payment, they are holding your money (and maybe more than they would have normally after the change) so the only change is in what they are being asked to deliver.