You can try removing the pads and prying between the pistons & rotor to retract the pistons. Use care to avoid damaging them. If you can force both pistons to retract, put the pads back in and step on the pedal several times to work them back out. Sometimes that will free up a piston that is only slightly stuck.
Probably just me, but I would be very hesitant to pry against a rotor as it would be very easy to bend it a few thousandths.
Also, when replacing the pistons, I would spring for SS ones( along with dot. 5). I know that Moss sells rebuild kits with steel pistons for about $27, but would rather do the job once and forget it. Maybe moisture isn't as much of a problem in So. CA compared to OR.
Berry