Back off the rear wheel axle adjusters an equal amount, let's say 3 full turns, the belt should come off then. After the rotor replacement and wheel reinstall, just snug the adjusters up the 3 full turns again and you should be back where you started from. Recheck the belt tension when you are done just to be sure of the proper tension. Don't forget to use heat on the head of the bolts on the rotor to soften the red locktite that is used on those bolts or you will probably end up stripping the heads out trying to remove them. Use red locktite on the new bolts when you install them if they don't have the locking patch on them already and torque thenm in a cris cross patern to avoid warpage. Also pay attention to the orientation of the wheel spacers on the axle.
The caliper should be no problem to stay where it is, if you need some extra room, you can seperate the pads a bit by opening the bleeder on the caliper as opposed to pushing the fluid back into the system. Just be sure to close the bleeder before you release the pressure you applied to the pads to move them back or you will get air in the system.
If you don't have a service manual, it's a good investment .