hello,
tbh, your better sending both away to get refurbished at the same time
as your now going to fit a like 100% working caliper, brake and handbrake
then a used probably slightly sticky or not working 100% on the other side
so probably get uneven braking, uneven brake pad wear, uneven handbrake etc
your welcome
also you could try unseizing the seized piston, remove all the pads from that sezied caliper, pump the brake pedal hard,
might free it up,
whilst your changing the caliper and bleeding the brakes anyways

note, leave all the other brake pads in the other calipers etc
later
glen