So I have a gen 4 g19 and am looking to buy a complete g23 upper so I can just swap uppers to go from 9mm to 40 sw. Living in NJ is a pain to get another pistol so i figured i'd get another upper instead and have a 2 for 1 pistol. Does anyone know if I have to buy a gen 4 23 upper or will a gen 3 upper work on my gen 4 g19 frame? Thanks!!!