the hotel cannot cancel reward certificates and probably has to call to have them modified, the same as you can.

it will likely be an issue you will have to spend some time on to get resolved.
plan on being patient and the appropriate amount of time to get it taken care of.
a few minutes will not be enough (it could take 30min -1hr), depending on what issues they will run into.

it's no smooth transaction to change a reward stay when the hotel has now sold out.