I think it depends a lot on the gun and the parts used. I have reconverted 17th century flintlocks from percussion to flint. Certainly the majority of its uselife was as a flintlock so I do not see it damaging to the gun's history to restore it to the way it was originally utilizing the correct parts. Now if you are converting using the wrong parts it just is a monstrosity and looks nowhere close to correct but done right, it looks better, is more correct than the conversion, and is worth more. To be sure the age of the gun has a lot to do with it. I would never consider reconverting a US musket of the early 1840s because it certainly is more correct in percussion than the half dozen years it was a flintlock but for a Revolutionary War era or earlier gun I have no problems with converting it back to the way it should be. Same with converting later guns of a certain significance. Take for instance a French AN IX, made for the Napoleonic Wars, converted later for hunting or whatever. The biggest significance of its life is as a flintlock under Napoleon, no problem reconverting that (not to mention there were 9 million made, original parts are available and they are not exactly rare).