Never understood the whole ridges vs non-ridges issue. We don't need the stupid reason that they used in ENT to explain it, either. There's a much simpler explanation.
Here's my take on the situation: We see two older, ridge-headed Klingons at a bar. In the background, a bunch of younger TOS-style Klingons are sitting at a table. One of the old Klingons looks back over his shoulder at them, then nudges his partner and says...
OLD KLINGON 1: "Would ya look at those dumb kids. No respect for the old ways. The way they dress is dishonorable. And their hair's so damn long in front, you can't even see their ridges!"
OLD KLINGON 2:"Relax, they're young. No different than you or I at that age. Our parents hated the way we dressed too. And tell me you never painted your ridges just cause you knew it'd piss 'em off even more."
OLD KLINGON 1: "Well..."
OLD KLINGON 2: "Of course you did. We all did. Young folks rebelling against the norm is nothing new. It's just a fashion thing. A Fad. And just like painted ridges of our day eventually went out of style, so will this. Sooner or later they'll come around, grow up, realize just how stupid they look, and cut their damn hair. And then the generation after these guys will adopt some new equally stupid fad, and it'll be these guys' turn to bitch and moan about it."
Thus, the TOS Klingons always had ridges, but we simply couldn't see them because of the way they wore their hair. No plague, or genetic tinkering, or anything more complicated than a simple, relatively short-lived fashion statement has ever been necessary to explain it.