If you are just throwing a set of pads on it, push the piston back in, remove the caliper, and swap pads. You should use some "brake quiet" while you are at it, but then you just put the caliper back. Inspect everything else, and If the lines are all cracked, you should change them. Do not let the system drain, because it is a PITA to re-bleed the master cylinder. If you opened the system, bleed it, and off you go. If you do not open the system, you do not have to bleed it.
A couple of notes of caution, if you are pushing the piston back in, make sure the master cylinder is not full of fluid, and can spill out. The fluid from the piston will go back into the master cylinder reservoir, and fill it up quickly. There is about 2-4 ounces of fluid in the piston. The brakes will gravity bleed, and you can close and pump if necessary. If your fluid is really dark, you may want to flush the brakes. You have to start with the most remote first (actually the rears are equidistant, because the single line goes to the center of the axle), and work your way to the closest.