Smith and Wesson will not change the caliber of one of their production pieces. They will not install a new cylinder in another caliber or a different barrel in another cailber, at least they did not do that in the past.
They may change the caliber of the entire piece with a new cylinder and barrel, as the barrel identifies the caliber, but they won't install just a different cailber cyclinder or a different caliber barrel.
I learned this from them when I asked to have my 29-2 rebarrelled with a light tapered barrel from a Model 24 44 Special. They said - Nope, that would change the caliber (marked on the barrel) even if you continued to shoot magnums and the M24 barrels were not made to the same specs as the M29 barrels. HTH.