Previously, I had reported that the .40S&W magazines would feed 9mm ammo. I found out this past Sunday that such is not 100% accurate. To clarify, during my first range outing, at least one of my mags loaded and fed 9mm ammo with no trouble. Last Sunday, one of my mags would not even entertain the thought of holding just one 9mm round much less feeding it. I suspect that there is a slight tolerance variance in the feed lips that makes one do it and the other not.
In short, if you want to do the multi-caliber barrel swap conversion, you will probably want to go ahead and invest in some 9mm magazines to go with the extra barrel. I'll be doing that soon.