I don't know the rules for IDPA or IPSC, but USPSA Production doesn't allow barrel switches. Thus, they would require a complete upper change, not just a LWD barrel. The porting holes in the slide represent a cut-out to reduce slide weight and I could imagine that being frowned upon.
I want to shoot one caliber for now. I want the g21 for ssp and my 1911 for cdp.
"A strong body makes the mind strong. As to the species of exercises, I advise the gun. While this gives moderate exercise to the body, it gives boldness, enterprise and independence to the mind. Games played with the ball, and others of that nature, are too violent for the body and stamp no character on the mind. Let your gun therefore be your constant companion of your walks." Thomas Jefferson