I would think that once you have arrived at the resort and taken possession of a UHP, then you are stuck with it. However, if you haven't actually started the vacation yet, there might be some way to cancel the UHP portion, and buy an AP instead; I'm thinking that it would depend on what sort of room package you purchased. Special packages like the FTP would prohibit switching ticket types, I'm sure.
You might want to try this question on the WDW resort board, since UHP's are only available to on-site resort guests.
Edit: UMP, UHP, LOSP, etc. I have trouble keeping up with the constant name changes for those type tickets.