The two cartridges are completely different dimensions and geometry. you need will need to modify the extractor, ejector, feed ramp, magazine, barrel ,recoil spring etc.
You can do a conversion but it would necessarily include a slide, barrel, springs and magazine.
Some models of 9mm will interchange a barrel only swap or minor parts, mag and barrel.
The 9mm and .40 S&W case heads are almost identical and therefore easy to swap out without gunsmithing (drop in) but on the .45auto the case dimension differences are going to create more parts out of alignment. Look at hte new polymer pistols for a easy swap candidate.