And there is no need. 1 oz of shot performs exactly the same from a 12 as a 20 with exactly the same recoil. It is the ammo, not the gun. There are 12 guage loads readily availabe that duplicate 20 guage performance and recoil.
The purpose of buying a 20 is to have a lighter, easier to carry around gun. Even if it were possible to convert a pump or semi to 20 you would still have the 12's weight with none of the advantages of a 20.