He played SS before that, so no, it's not "on the fly." If you have experience playing SS, a switch to 2B isn't difficult. A move to 3B would have been more difficult, and that's been done with ease by many.
Soriano is probably like Cabrera in that he teeters on that "are you good enough" when it comes to playing a tougher defensive position. Soriano mashing the ball at 2B allows the Yankees to get mashers in left and right field. They probably thought they're better off doing that, rather than putting Soriano in the outfield and trying to find a better fielding 2nd baseman who could hit.
Regardless, Soriano has the ability to play 2B, which makes him better than what...80% of MLB position players at being able to play 2B? That adds value. That's the point re: Beltre.