Re-shaping the grip frame requires nothing more than the proper template and grinding. My memory is that Brownell's or Numrich used to sell the templates for doing this, so you could try looking at either of these sites. However, square butt 686's will become increasingly rare as the years pass, so I would be inclined to leave it as it is now, especially if it's serrated.
If you're looking for grip options, there are a lot of excellent grips available for the square butt grips frame and I prefer shooting with the square butt profile even on my guns with a round butt grip frame.
Personally, I prefer shooting with rubber grips, I like the extra traction rubber provide. For a square butt grip frame shooting mainly 38 caliber in your 686 my choice would be the Pachmayr Gripper Pro because I like the fit in my hand. For a lot of 357 Magnum shooting I'd look into the Pachmayr Decelerator for the square butt grip frame, it'll provide a bit more cushion. As for Hogue rubbers, love the 500 Magnum monogrip but it's round butt only. The Hogue standard monogrip is too small for me and I've replaced my standard Hogues with the the Gripper Pro.