I'd wait and try out some magnum loads first, before changing anything.....as another poster has indicated.
If the revolver shoots 240 grain magnum loads well, with the current rear site blade, then it's correctly targeted.
If that fails, as yet another previous poster suggested, a rear sight slide change may be required.
S&W K, L, N rear sight slides are available in .106" .126" .146" .160" .196" and also require a new adjuster screw and nut.
Smith & Wesson Parts sells those, as well as MidwayUSA, Brownell's, and other gun parts suppliers.
BTW: the sight slide is measured from top to bottom of the slide, (not the notch itself), as it would sit in the sight bracket.
Hope this helps!