Got a buddy that works for Walther and they use same frames for 9 and 40 but he's not sure on the m&p. May just be better off taking apart and swapping or ordering another trigger.
I would be the 9/40/357 frames and slides are all machined to the same outside dimensions, but I don't know about the inside dimensions. I was just searching trying to find that thread ont he SW forum, and I am reading conflicting info on wether you can swap slides on 9 and 40 frames. Some folks say yes its the same, and others say there is a few thousandths off and the ejectors are different