Someone 'splain me why there is a valid concern about how the switch from being a position player to being a DH has a negative effect on offensive performance, yet changing from one position to a new position is looked at as no big deal.
In the former case, the player has even less to worry about. The defensive aspect has been removed from the equation and all he has to do is concentrate on hitting. In the latter, the player has to concentrate on learning to play a new position as well as continuing to focus on offense.
Not sure that I buy the "sitting-in-the-dugout-and-not-playing-the-field-saps-your-readiness" argument either. Edgar Martinez, Paul Molitor and Dave Winfield didn't seem to have any trouble with it.