1911 slides come with two different breechface sizes, the .45 size, and then a smaller one which accommodates 9mm/.38 Super/.40/10mm. You could use any of those calibers in your slide. You would need to change the recoil spring, barrel, and magazines, of course. The .45 and "small" slide also do not share slide stops and ejectors, but you don't need to worry about that since yours are the "small" one already since you are starting with a .38 Super. The barrel needs to be fit, usually. It is not "drop in."
A combination 9mm/38 Super would be a nice set-up to have.
To convert it to .45, you would need a new slide, barrel, slide stop, ejector, and mags.