You CAN unfreeze it. But if the piston is stuck, then more than likely you have a bad caliper, and unfreezing it will have it get stuck again almost immediately. It's what happened to me personally.
I recommend just going to NAPA and ordering the calipers. And then give them your old ones for a core exchange refund.

Also, how do your rotors look? My calipers were so bad that I had a huge lip on the rotors, so I have purchased replacement brake disks as well to go along with my pads, brake lines, and calipers.

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1998 Ford Escort ZX2 5 speed - 279,000 miles My Daily

1992 Mercedes 300D 2.5 202,000 - Pure junk
2000 Mercedes E320 Black - 136,000 miles - Needs repair

Don't forget to grease the screw and threads on the spring compressor.