I think the notion that the S&W Modification to M586/686's could lower value comes from the fact that the Safety Conversion Ruger does to their
Old Model Blackhawk does have a BIG effect on value. Unconverted OMB's sell for way more.
But the comparison isn't fair. The Ruger conversion changes the trigger pull and other aspects of how the gun functions. The S&W conversion is totally invisible to the end user, and doesn't change the human-to-gun interface in any way.
In fact, if they weren't marked with a "M" you'd have no way of even knowing the modification had been done. Has the market price of unmodified OMB's created a feeling in the gun community that ANY recall or modification to a gun lowers it's value?
If we ever do see a large price difference between modified M586/686's and unmodified guns we may have RUGER collectors to blame.