dahermit,
I've had a couple occasions where for the life of me I could not get a rear sight to budge with a brass punch.
I ended up taking drastic measures.
I found a way to position the slide on a flat smooth concrete floor with a piece of cardboard and paper towel between it and the floor. Then I used a wash cloth or something to put over the rear sight and the slide. Then I stood on the slide with one foot to keep it from budging.
Then I used a good thick steel bolt that was laying around as a punch (my brass punches were becoming mutilated from my previous efforts) I put the punch at the bottom of the sights base where it meets the slide so that it was kind of locked into the corner between the two, then I used a regular heavy wood working hammer to tap on the bolt head, harder and harder and harder, checking the bolts position on the sight and checking the rag/washcloth to make sure the punch wasn't cutting through it.
Finally I noticed that I had knocked the sight over enough to be sure I had actually moved it.
Using some penetrating oil and possibly some heat are good ideas, but what did it for me in the end was basically steel on steel with a decent amount of impact and mass with a rock solid base.
Somehow I managed to do all this without marking up the gun.
Have someone else stand on the slide while you keep the rag, punch and hammer in alignment if you need to. But I really would recommend using a thick and wide steel punch.
Good luck buddy!
PS... Just a bit more elaboration to be sure I'm communicating this effectively. I really felt like the floor was an essential part of it all. I tried using a vise but there was too much flex and bounce from the vice and table and all that. I guess it depends on how solid your vice/workbench set up really is, but you can't do much better then a solid concrete floor. Having a good heavy punch is essential too, as well as a solid hammer. If you have flex or bounce anywhere you may not attain the solid deliberate shock needed to break it free. Just use whatever you can to protect the slide and sight from marring. I feel like regular box cardboard is good because it will cradle the slide and is soft, but it will compress so that it ends up providing a solid base after a few hits. A paper towel is good to protect the gun from the potentially rough cardboard. If you want you could try thinner cardboard from a cereal box, and that actually might be what I used if I remember correctly. Then good luck with whatever you use between the steel punch and sight, just make sure it will allow the energy to transfer but will still provide enough durable protection between the two bits of steel.
Oh yeah, one more thing. Once I found that I actually COULD move the sight, I went to the range to make my final adjustments. The formal range that I usually go to was ok with me removing the slide and barrel and all that and hammering on it on the floor just behind the firing line. I kept it up, resembling and firing three shots from a bag until I was satisfied that the sights were centered. Both those guns shoot great now, right to point of aim, and I can be sure they will stay that way as it would take an act of God to bump those sights out of alignment.
And yes, I was proud to have saved myself the time and money of taking it to a smith. Just be patient, careful and deliberate and I'm willing to bet you can get that rear sight to move without screwing it up.