I was unaware it was this simple to swap calibers in the Shield, and am happy to know that the .40 slide will work with 9mm. I will be buying a .40 Shield and barrel/mag for 9mm. I see little point in getting a .357 barrel, as I don't particularly like that caliber.

I have a Colt M1911 (yes a real WWI M1911) in .45 ACP, and for over 10 years used the same frame for .45, 9mm, and .38 Super. However, it is necessary to have a slide for 9mm and .38 Super (.38 Super and 9mm use the same slide), plus a .38 Super magazine (which also works fine with 9mm), a 9mm barrel, a .38 Super barrel, and an ejector for 9mm/.38 (same for both). A heavier strength recoil spring is also needed for the .38 Super.