Actually, that theory ceased to hold water at all with DS9's Blood Oath, which showed Kor, Kang and Koloth, played by the same actors as in TOS, as ridgeheads; and Trial and Tribble-ations, which explicitly showed TOS Klingons alongside a time-travelling TNG-era Klingon and implied that whatever caused the change is someting the Klingons find embarrasing. For a while I went by the theory that the change to smooth-headed physiology was inflicted by the Hurq (certainly it was later than the time of Kahless) and was only reversed about 2270. Now, it would appear that the actual change was made in the 2150's, possibly as a result of the same genetic engineering done on the Suliban (those who haven't seen Enterprise should ignore that last...)