Joe, as far as I know, with the M&P the frame is the same, the slides are different. I've got a .40 S&W recoil spring assembly installed (new one) in the .40 slide/barrel set up. When you pull the slide off the barrel and recoil spring stay in the slide unless you remove the recoil spring and barrel.
Only thing I had to modify was the magazine(s). They are slightly different. Got out the calipers and measured/took notes and decided on the path to get what I thought I wanted on that 10 round magazine. First very short test got me part of the way there. I think it's ready to run now with that 10 round (8 .40 S&W rounds) magazine.
Of course, as often happens, I sat down with the 10/8 round magazine when I worked on the follower some more and came up with a different method to try on the 13 round 9MM magazine to get it to run .40 S&W. Much quicker/easier and we can argue about how pretty it is (I don't care if it works). I'll try it the next range trip and shoot more ammo, too.
I just get a kick out of doing things others don't think of or think can't be made to work. Part of the reason one of my CZ75 Compacts (not easy) and my CZ75B .40 (pretty easy) now look/operate like CZ85B pistols.
Timetofly, I also bought a .357 SIG threaded barrel for that .40 S&W slide. Didn't even try to shoot it yet. I did buy a compensator for the end of it though. If it feeds .40 there's a good chance it'll feed .357 SIG. Not sure how it'll shoot though. If it shoots good enough I'll be looking for another .40 S&W Shield slide to keep that .357 SIG barrel/compensator installed in.
That .40 S&W Shield Plus is just going to be a range gun for awhile till I shoot it enough to think it's reliable enough to carry.
Oh, picture of the .40 slide, with the .357 SIG threaded barrel and compensator on the 1.0 Shield frame. I think the comp. is made by Sampson. There's a thread protector with groove in the outside that screws down hard/tight on the barrel (supposed to use some thread locker, too) and the the set screws thread into the comp with the tips biting into those rings to lock the comp in place. If I end up using the .357 SIG set up I'll probably try to get the slide and comp. cerakoted so they look like they go together. Two tone will be fine for testing purposes. S&W doesn't make .357 SIG Shields/Shield Plus pistols either.

Batteries are out of the light/laser because it drives the darn cats wild when I grab the grip and the laser splashes on the wall or floor or chair.