to cancel an onsite room, do it 5 days prior so you can get your deposit back.
if you are only going to change hotels, call and see if the room you want at the different hotel is available for your dates.
if it is, then book it then have the TM cancel the other ressie.
if you only stay one night then you would pay the one night room rate at the time you book it.
the canceled ressie refund would take about 5 days for it to be posted to your credit card.