I'm not clear if the caliper is frozen or just extended as the pad wore. If it's just extended you can push it back in with a big C-clamp. I have one I bought many years ago just for this purpose though I keep finding other uses for it. Sometimes it can be difficult to push the piston in even on a good caliper. I often find that I have to turn the screw on the C-clam a little, wait for the pressure to decrease a bit, turn it more, repeat.

If you have multi-piston calipers just leave the piston-side pad in place and push all the pistons back together.

If it's seized I agree with the guys - cheaper, easier and more permanent to install a rebuilt unit.

Dan