That gun would be mediocre in all calibers but best in none. I'd stick to shooting and training with the gun made for its caliber.
That said, a Glock 21 can be easily made to shoot .45, .40, .357sig, 10mm, and 9x25, all of the big auto cartridges, with a barrel and slide swap.
edit - Now what I would like to see is a Glock carbine with a quick change caliber capability like the XCR. It'd need to swap the barrel, bolt, and insert a spacer to use the small frame magazines.