I've checked out of a Hyatt in the middle of the stay because I didn't like the hotel. I was only charged for the nights that I stayed; no additional fees were charged. I immediately checked into a different Hyatt (this was in Seoul; I left the PH after 2 nights (IIRC) of a 9 night stay and checked into the GH)

I'm not sure if this would always be the case. I'm not sure if this is easier or harder than changing the reservation before you check in, but in my case it worked just fine.