Yes you can convert your G23 to .357 or 9mm.
For .357 you will only need a .357 barrel barrel (stock or aftermarket) and you can use same magazines.
For 9mm you would need an actual conversion barrel (LWD and others make them) and G19 mags as well.
For .357 you will only need a .357 barrel barrel (stock or aftermarket) and you can use same magazines.
For 9mm you would need an actual conversion barrel (LWD and others make them) and G19 mags as well.