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.
I'll have to respectfully disagree with the above statement.
Frames are different between calibers, the frame ramps are cut diffferent in angle and depth.
Now if you mill the frames to accept a ramped barrel changing calibers will be easier to do, as the correct ramp will be built on the barrel.