Like Agalindo says... you can go to a smaller caliber barrel with a pistol made for .40/357Sig, or perhaps from a .45ACP/.45GAP to a .40/357SIG sized round, but the breechface of the 9mm will not accept larger calibers, nor will the hole in the front of the slide that the barrel comes through. 9mm conversion barrels are thicker than stock 9mm barrels in order to compensate for this difference when fitted to a gun which was manufactured as a .40 or 357SIG. I'm not even sure if eventually they will have .40/357SIG barrels for the guns that came as .45ACP's, but it might be possible. I don't think you will be able to reliably go from .45ACP to 9mm though without some other changes being made... principally to the extractor and ejector, as the .45ACP is so much wider at the base that the extractor would not be able to close on the rim of the smaller 9mm round, and the ejector might miss it entirely and not fling the empty case free of the breech when cycling. Feeding on the overly large .45 breechface would also be a problem as the head of the small 9mm case would be free to "move about at it's leisure" so to speak. These same problems might apply to the .45 to .40 conversion to a lesser extent. I would have to take a lot of measurements, as I'm sure the makers of conversion barrels already have, to see if this is feasible.
I, Procrastitron will destroy evil... eventually!
Strangely, a lot of folks who do not believe in evolutionary Darwinism seem to believe in social Darwinism... go figure!
Let's add this: Congress shall make no law which limits the type, caliber, capacity, or rate of fire of any arms the People have or want to have.