The central problem is a statistical one. All TOS Klingons without exception had flat foreheads, whereas all Klingons in TMP and later without exception sported the characteristic ridges. The latter include the Klingons who have traveled to the Delta Quadrant (VOY: "Prophecy") and have probably embarked on their long journey just at the time of TMP. The appearance of Kirk's former adversaries Kang, Koloth and Kor in DS9: "Blood Oath" makes the explanation even more difficult. They must definitely have undergone a metamorphosis. We also need to wonder why the Kahless clone in TNG: "Rightful Heir" looks like contemporary Klingons and not like the flat-headed fake Kahless in TOS: "The Savage Curtain". Maybe the TNG Kahless accordingly altered to look like present-day Klingons? The same may apply to Worf's statue of Kahless and Morath fighting who had ridged foreheads as well. If the Klingons had not always looked like that, maybe they were adapted in the same fashion as depictions of Jesus Christ from different eras show him in contemporary clothes of these times, probably to allow a better identification with the historical figure.
