Originally posted by Lowell Goudge from my perspective, I think although the aperture and mount are different, the two main optical groups would be identical, and you can remove one optical group from th eback, and the other from the front and just swap them.
in many cases the aperture movements and linkages are backwards. so just changing mounts can't work.
Whether is is that easy depends on the lens. In a prime lens, swapping the optical elements/groups would probably quite straightforward. In a zoom lens, that's often not so easy, because you need to disassemble a lot of the complex mechanics to get to the moving optical elements inside.
Ben