The reason for the modification was to address the issue of primer flow back into the firing pin bushing, which potentially could lock up the cylinder. If this happened it was possible for the cylinder not only to stop rotating, but be very difficult to open.
Apparently, there were some very soft primers being used by either some handloaders, as components, and/or in new loaded ammunition, not necessarily from any major ammunition manufacturer, either ... S&W can't control the quality of the primers used as either components or in loaded ammunition.
This was apparently only a possibility, as it was explained to us, with the particular bushing and firing pin nose used on the new L-frame design.
To prevent the possibility of lesser quality primers causing this condition to occur in their 686 revolvers, S&W decided it was best to modify all existing revolvers with a redesigned firing pin bushing and firing pin. We received the parts and all of our 686's in inventory were so modified.
As a side note, we never experienced this problem with any of our 686's, and I've never personally spoken to anyone that did, either. At that time I was still using a lot of handloaded ammunition for range practice, and a lot of "budget" or small manufacturer "gee whiz" ammunition, as well.
S&W wanted to prevent the possibility, and that's fine with me. I'd rather have them do that, than simply ignore the possibility, and then later claim it was an ammunition problem and not a problem with their revolver, so it wasn't their problem ...