Replacing one manual safety body assembly with another one usually requires replacing and fitting a new sear release lever in the frame. This is an important part of the decocking function, as the tolerance between these two parts sets the "timing" of the decocking, which is critical for safety reasons. (Having a gun go BANG during decocking is not good, and having a gun where the hammer won't decock at all when the manual safety lever is fully lowered isn't good either.)
Fitting a new sear release requires filing the foot of the lever, while carefully maintaining 2 critical angles of the foot's surface while shortening it so the proper decocking timing is achieved.
This used to be done by eye-ball during manipulation of the manual safety lever and observing hammer fall, but in more recent years armorers were told to use 3 different "gages" (the non-cutting ends of numbered metal drill bits) to check for the proper timing and how much a lever might need to be filed for any particular gun.
Yes, the process requires some knowledge of detailed disassembly & reassembly of the frame, as well as repeated assembly and disassembly while the lever is being filed and the decocking timing is checked (often after each single file stroke, as you get close to the right tolerance). It can be a tedious process.
It used to be even more tedious when the levers were cut really overly long for older guns (more generous tolerances in guns in those days), but as newer guns were being produced to tighter tolerances the newer levers weren't being cut quite so overly long.
This is something that really ought to be done and checked by a factory tech, a smith familiar with repairing 3rd gen S&W's or someone trained as an armorer (and preferably has gained some experience in doing it).
