Originally Posted by Ballardw
Since you mention more than one manufacturer, it is important to note that this statement is not true for all makes and models. For example, in the 1911 family the 45 frames will not work with 9mm/38 Super and such calibers as the ejector will not contact the case properly.
There is no difference in the frames. The ejector is not an integral part of the frame and can be switched. However, 1911s use three different slides due to case head dimensions. The original, of course, is cut for .45 Auto. The second breechface cut is for 10mm and .40 S&W. The third is for 9mm and .38 Super. There are also differences in the magazines. You cannot simply swap barrels on a 1911, unless you are switching between 9mm and .38 Super, or between 10mm and .40 S&W. But, with a change of barrel and slide and ejector, you can go back and forth between 9mm and .45 Auto. I have one pistol set up as such a switch hitter. I omit the ejector pin to facilitate swapping over.