I Have a M&P 2.0 in 9mm I has seen the 40 to 9 conversion done as just the barrel and as a slide and barrel with no issues. I contacted S&W customer service directly and posed the question of can it go the other way (9mm to 40 with a 9mm frame) if you are using a slide a barrel combination. They stated that you can with no issues and no modification to the slide, but you do have to keep the frame styles together compact to compact for example. You can on the other hand run a longer slide than was originally purchased for example you can run a 5" 40 on a 9mm that had a 4.25" from the factory.