As soon as you start swapping slides it's no longer a .45 ACP and the thread title specifically states ".357mag vs 45acp".
But just for the sake of argument, let's allow that. So, you're in the field and figure you need a .22, did you bring your extra slide and magazine(s) along? Or maybe just go from full power loads to creampuff loads - did you bring the proper recoil spring?
Same situation with a .357 revolver. Now you can't change the caliber but you can load anything you want with just what is currently in the holster. Dump the rounds in the cylinder, load something else and you're ready to go. No swapping parts or carrying different magazines. About the biggest modification you'd need to do is adjust the rear sight a few cicks, and we're all excellent and experienced handgunners, aren't we? So we know exactly how many clicks it takes going from one load to the next.
Heck, if we want to go down this road, a T/C Contender kicks the ass of both of them and is the most versatile handgun in the world since it can handle both .357 and .45 ACP rounds - .22 rimfire to .357 to .41 and .44 to .45 Colt or ACP to .444 to .30-30 to .45/70 or anything you can think of, all in "one" gun. It can even be a .410 shotgun. Just carry a pack of pre-sighted spare barrels everywhere you go and you're set for everything from chipmunks to brown bears.
If we're just sticking to one caliber, and an unmodified gun...A 1911 can be setup to shoot most any light load all the way up to .45 Super with no change to the gun whatsoever.
