FF, be careful when you check for seized pistons when the caliper is off the bike.
As Aldo said, put something between the pads to stop the piston coming out too far.
Things to watch :-
Kep an eye on the fluid level - dont let it get down too far or it could suck in air, but dont top it up when the pads are out, could overflow when new pads are put in.
All pistons will not come out together - some take a bit more effort than others to get them moving, you will have to stop the easy ones to try & get the others to move - from this judge if they are seized or partially seized.
Dont let the pistons come out too far, they can get past the seal & will mean a re-build & bleed.
BUT- MOST important, give the lever a good squeeze a few times when you put it back together. If you dont, you will hit the first bend or corner & **** yourself, it certainly wakes you up if you dont.