The Klingon Explanation
From season to season, and series to series, the Klingons have drastically changed their appearance.
At first, it was just budget limitations (hence the smooth foreheads), but as ridges and whatnot were added to their scary visage through the years, the writers took the lazy path and said it was just a “mutation” that changed them. There’s even an episode where Warf says, “We don’t talk about it.”