I should say that I never really cared about the change in Klingon appearance. To me it was obviously what it was -- a change in makeup.
It wasn't an issue until Worf had to get all meta in Trials and Tribblations and explicitly acknowledge the change as something in-universe.
At that point, it was begging for an in-universe explanation.
I disagree. I think it was handled BRILLIANTLY in the episode. "We do not speak of it" (exact quote escapes me at the moment). They had to acknowledged it, as it was almost painfully obvious with both Klingon versions in the same scene. But I absolutely loved how they acknowledged and side-stepped it in almost the same sentence, while leaving every fan's theory still in-play. That was where they should have left it.