Don't play around with it online but call the Priority Club or as Nicholas75 suggested even better the Ambassador service center instead. Do not call normal reservations as they can't do anything and the hotel is useles too as they can shorten your stay but will keep the points and not return them (at least that is what the hotels told me in 2-3 cases).

Just try your luck and call up PC / Ambassador services and let them know that you need to shorten a stay by 2 nights. Now your chances are mixed. I had some agents who did this without a hitch and returned the excess points even though there was no more award availability on the days I wanted. (I mean you are holding a room already so why does award inventory even come into play?). Some others claimed that there is no award availability so they cannot change the reservation at all, only cancel it. This is stupid but seems more common these days. Particular tricky if it as a PB hotel, in that case I would just suffer the loss of 10K. If it is a normal redemption rate then either pay or look for another property. Sorry - not that many choices.