They do make a conversion for the 21 to be converted to 10mm. You swap the barrel (conversion barrel, not a G-20 barrel), magazine and extractor. The .400 Corbon is based on the .45acp case, so extractor and magazine are fine. I have not seen a G-21 to .40 or .357 sig.
Other than the 22 to 9mm conversion, are the converted guns essentially the same as the factory chambered guns?
No, only the .40 to .357 sig conversions are close. You can swap factory barrels there, but magazines are slightly different between .40 and .357 sig. (most people report either works)
The G-21 has a lighter slide and wider breach face than the G-20, plus the extractors and ejectors are different, but can be swapped.
The G-20 is a much bigger gun then the .40/.357 Sig guns, but the conversions work well because of the common case head dimensions. My G-20 .40 conversion was really nice. Very soft shooting and more accurate than a G-35 I had at the same time.
The conversions are basically for range use, not to be relied on for self defense because of the subtle differences between the models.