my suggestions are that once you have pulled the old pads to use an old tooth brush and some air to clean the calipers and pistons before you push them back in. Follow that up with some break cleaner and a shop towel, then push the pistons back in. Reminder brake fluid is not kind to plastic, cover the bike below your master until you are buttoned back up.
If you have gg and replace with HH I would suggest replacing them all at once (HH are more aggressive) and then pump your brakes up before you ride it. Tons of threads on it.
switch inside and outside rear , the inside will wear much faster.