IIRC, 686's in 4", up thru -4, were not pinned on sights. I believe that started with -5.
If I'm wrong and it is pinned in, you would need to order the sight, knock out the existing pin which would then free the blade. Install the new blade and drill a hole thru the blade itself. If it's pinned, the hole will already be there on the ramp. Smith, to my knowledge, does not ship a predrilled blade.
So...if you have a drill press it's an easy job. But as previously noted, it's not like you have to take out a new mortgage to have your local smith perform this action.
If it's not pinned, that will be a tad more expensive, but perhaps not hugely so. And you would then have the option of selecting a HUGE number of front sight replacements.
Hope this helps.