From a Wikipedia article on retcon.
When Star Trek: The Motion Picture was released, Gene Roddenberry claimed that the radically different appearance of the Klingons in the film was how they were always supposed to have looked, but they didn’t have the budget for it in the 1960s. Years later, an episode of Star Trek: Deep Space Nine featured Klingon characters from the original series, made up to fit the new look. However, the later episode “Trials and Tribble-ations”, used footage from the original series with old-look Klingons; Commander Worf acknowledged their different appearance, adding that it is “a long story” that Klingons “do not discuss with outsiders”. It is now rumored that an upcoming episode of Star Trek Enterprise will provide an explanation for the difference in Klingon appearance.