I just went through something similar with an old P228 police trade in. Getting the rear sight loose was a real bitch.
I sold off most of my SIG's about a decade ago, and with them, the MGW sight tool I had for them. I REALLY missed that a couple of weeks back.
I replaced the night sights with a new set, and used a hammer and punch. Major PITA too, I might add.
I dont know if they put Loc Tite on the sight when they did them or what, but it sure felt that way, and I had to beat the hell out of it to get it off. You may have to do the same if the tool wont break it loose.
I replaced quite a few sets of night sights on SIG handguns over the years, using that MGW tool (it did both front and rear sights) and I cant remember ever having a problem getting the sights off, or on with it. Certainly nothing like that.
At least if you have to beat it off with a hammer and punch, its the old sight. When you go to install the new sights, if the sight only starts into the dovetail very slightly, run the base of the new sight over a sheet of fine emery on a flat hard surface for a couple of swipes, until the finish just comes off the bottom. It makes getting the sight on a lot easier, and with less effort.
Dont get too carried away and take to much off, or youll end up with a loose sight. A couple of swipes is usually perfect.
I keep hearing people say they always use the hammer and punch method to do the swaps, but I have to assume they have never used a tool, because their method is a big PITA. If I have to go through that again, Ill just buy another tool. Its the only way to go.
The other great thing with the tool is, any adjustments can be done at the range, with no effort or drama, and with precision.