Re: XDm 40 to 9mm
Did a bit more "reading" on this, so take it for what it's worth. There's conversion barrels out there but they've run into reliability issues due to the positioning of the extractor on the smaller size casings. If you're planning on doing this to save on the cost of ammo, you would probably be better off putting that money into reloading instead.
XDTalk has a few threads on the subject.