There is no published policy with respect to changing non-AC redemptions, so the assumption should be made that your ticket is only valid for travel on the actually ticketed flight in the actual class of service unless IRROPs exist. And is literally worthless otherwise.
My personal albeit very limited experience is that United applies the same policies to AC/AP tickets as they do to UA/MP tickets operationally, ie: they are indistinguishable in their IT systems (absent further digging), but staff definitely know that you are on reward travel.