I’m not sure about your 2nd assertion. A while back there was a thread where someone picked up a SAA which had been well-used, with worn edges, etc. it was subsequently engraved (likely in the last 30-40 years) and then refinished. I didn’t think all the glitz did a thing to enhance the value of what was likely a well-used, but not abused, 1st generation Colt.Off the bat, there are two ways I can immediately think of that an alteration would increase the value of a gun.
1. Undoing whatever someone did to lower the value to begin with. Such as refinishing it to the color it was supposed to be, finding period correct grips, removing rust without harming the gun, etc.
2. Engraving the gun.
The thing is, both options usually cost more than the end product is worth, but they do increase the value of the gun. Otherwise, usually changing anything at all outside of something not permanent will always bring the value down, as it takes it a step away from being 'original.'