No. A Gen4 slide will not work on a Gen3 frame, the part of the slide that holds the muzzle end of the guide rod is wider on a Gen4 slide than the corresponding cut-out on a Gen1,2,3 frame.
(with the G26,27,29,30,33,39 a Gen4 slide will work on a Gen3 frame and vice versa)
You would need a Gen3 G19 upper including slide, barrel, and slide internal parts. You'd also need to get a Gen3 9mm trigger housing with ejector to replace the .40 ejector in your G23 frame. As well you'd obviously need 9mm magazines.
You could get an aftermarket .40-9mm conversion barrel with 9mm magazines. You'd be able to simply drop that barrel into your G23 slide and start shooting 9mm, you wouldn't need to change any other parts. But it wouldn't be as reliable as a factory G19, you may experience the occasional failure.
Last edited by voyager4520; 09-17-2012 at 21:34..