If they ever show Klingons, I wouldn't mind seeing the ridged forehead only. Who knows, maybe in the 4th season they got an infusion of funding and upgraded their Klingons. Just as I hope that if they ever show a Gorn, they don't use that rubber suit. As far as explaining it, that's TV, e.g., the Darren switch in Bewitched. I really hate that virus mutation crap.