Is it possible to convet a Glock 23 into a 9mm by simply changing the barrel? Or does it require that the magazines be converted too? I've heard that on some brands of guns, the .40 mags can also feed 9mm without fail.
First of all, you need a conversion barrel, which Glock does not make. The 9mm Glock G19 uses a different ejector, firing pin, spring loaded bearing and magazines. I don't have my manual handy, so I'm not sure about the extractor.