To convert a .40 or .357 Glock to 9mm, all you should need is an aftermarket conversion barrel designed to make the 9mm work in the larger slide cutout area of the .40/.357 Glocks, and one or more 9mm Glock magazines that are the same length as your current Glock (G17 mags for full-size G22 or G31; G19 mags for a G23 or G32). In Gen3 guns, the springs are the same for 9mm/.40/.357 in the same size Glock; in Gen4 Glocks, there are lighter springs for the 9mm guns as they originally didn't function very reliably with same springs as the .40/.357 when using some types of 9mm ammo. You should not have to change the slide at all, providing you bought the correct aftermarket (non-Glock) barrel. The special barrel will take care of centering the smaller 9mm cartridge in the larger .40/.357 slide breech cutout area.
The more controversion topic that I have heard many different things about is converting a G22 or G31 to 9mm. I know that I need 9mm mags, that the .40cal/.357 mags wont work for 9mm (supposedly g17 mags work in the g22/g31?)
I've heard that all I need to do to change these to 9mm is get a 9mm after market barrel designed for a g31 or g22 (availible from lone wolf).
I've also heard that I need the barrel as well as a new lighter spring (a g17 spring maybe?)
I've also heard that I would need a new extractor and ejector in order for this to work.
And I've also heard that I would need an entire new slide.
So in order to shoot 9mm from a g22/31 what do I need?
This would be for converting the pistol for cheap range shooting so 100% reliablility for 9mm isn't necessary, I just want to be able to effectively practice and shoot it.