If it is in a dovetail and is a standard sized dovetail, you might be able to change it yourself with a brass punch and some care. The only hard thing on 45's I think, is the front sight that must be staked on. But alot of newer ones have the front sight in a dovetail too. If your rear sight is a nightsight though, it might be better to have it changed with a proper sight pusher tool. Makers of night sights are loath to anyone hammering on their sights. If you DIY it, be sure to carefully mark the center of the old sight notch so you can set the new one to the same windage. A brass punch should not put any permanent marks on the slide or a steel sight. I have used a punch to adjust windage on alot of pistols, therefore it should be just as easy to knock it all the way out with one. Just that sometimes the new sight might need a little filing to the dovetail for a snug fit but it should be able to start into the notch without too much force. And some aftermarket sights have an allen set screw in the center so that even if the fit is not super tight, you can locktite that screw down and it will stay in place. You might get a Novak style rear sight or Heinie or any of the smooth contoured combat sights.
