My highly uninformed opinion is nothing more than model change.
Glock makes it easy to follow by keeping the model numbers the same and telling you which generation it is. Look at how they change from generation to generation looking for what customers will find comfortable and usable, and then spend their money on.
General Motors for years claimed that the biggest mistake they ever made was in the Square Body C/K pickups. Fifteen model years, 1973 - 1987, of very little change. I worked at a Chevrolet dealership for a few years during that era, parts manager and service writer, and I had customers that would need to replace a hood, emblem, tailgate, grill, etc., and they would request the newest part. They could make their old truck look new for no extra cost when fixing minor body damage. Many customers purchase new for the new style. For fifteen years C/K owners didn't have that reason to buy.
Is it possible that Smith & Wesson was just making a small ergonomic change in the new model?