if the piston is not budging, use plenty of wd40 etc on the piston, all around it and over it and everywhere else aruond the vicinity

then you the foot brake to try and push the piston out, after carefully arranging bits of metal to stop the other pistons coming out

you will ruin the pads and discs by coating them in wd40, but seeing as you are changing them in any case then it's a small price to pay

make sure you get a rebuild kit for when it's off so that you can clean the pistons up and do the general refurb of the calipers before they go back on again

not too sure what you'd do if the piston is scored though, as the new seals you put in might be damaged with the pitting or let fluid past