Uhm...
The M&P is a K frame, the Model 36 is a J frame. I don't believe that they use the same barrel shank dimensions, so I'm not sure how that would work. You sure that it has a J frame barrel on it?
Value wise, unless the M&P was a really mint gun, or was for some other reason really collectible, it's value is really in being a user handgun. I'd not think that its overall value would be affected that dramatically.