Since you have to break the existing windage bolt to get the blade out S&W sells a rear sight blade kit that includes the windage bolt and nut. Instructions are in 500 Magnum Nut's Frequently Asked Questions sticky thread in S&Wsmithing: FAQ's While he suggests buying a small spanner wrench one can easily be made by filing a small flat head screw driver. Be careful to not loose the tiny detent plunger and its spring. Those two parts love to pup out and hide in your carpet. Brownell's is an alternate source for the kits and they sell the spanner.
You very seldom have to break the windage screw to change sight blades. I reuse them most of the time. You only need to break them if the crimp on the nut side is too big. That said, it is easier to just break the screw and use a new one. Finding the right entire assembly for an older gun can be difficult. Finding a new blade, screw, and nut is very easy. You can make a spanner for the nut out of a 1/4" hex screwdriver bit for a buck or 2.
Replacing the blade is reasonably easy, but be prepared to possibly lose the teeny tiny detente and spring as you back the screw out. By putting everything inside a large clear (food bag) plastic bag it helps keep the items captured for reuse. ( I still have a teeny tiny detente in my shop somewhere).