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.
No, they didn't HAVE to acknowledge it. They never acknowledged the V shaped brow ridge some of the Romulans had developed in TNG. They never explained they myraid of make-up effects used on the Andorians (as mentioned above).
You like the explanation for the difference in Klingon appearance. Hey, that's great. But it wasn't necessary to explain away. Well, I take that back. I can see that with Worf being in the episode then some explanation as to why Klingon Worf looks different than the rest of the Klingons had to be given.
But they cold have left Worf out of the episode. They could have done a number of other things. They had already shown Kang, Kor and Koloth with the modern looking ridges without ever explaining their difference of appearance.
DeForrest Kelly joked that the TOS Klingons were "Northern" Klingons and the Movie Klingons were "Southern" (or vice versa). Someone else joked that the reason Klingons looked different was that our TV reception in the 60s and 70s was just poor and they always looked like that. We didn't realize until we saw TMP.
We didn't "NEED" some elaborate retcon from Enterprise to explain everything.
^ I believe the only mention of a "bird of prey" in "Balance of Terror" was this:
KIRK: "After a whole century, what will a Romulan ship look like, Mister Stiles? I doubt they'll radio and identify themselves."
STILES: "You'll know, sir. They're painted like a giant bird-of-prey."
That's a long way from declaring "Bird of Prey" as the ship's official class designation.
And, if memory serves, no mention of "bird of prey" at all in "The Deadly Years" or "The Enterprise Incident." Were the Romulan ships featured in any other episodes?
The argument about the Klingon/Romulan birds of prey is also muddied by the facts that:
A: Starfleet Battles referred to the Romulan Ship as a Romulan War Bird. All their ships were referred to by some avian designation. I think FASA did the same thing with their games. Apparently enough people and Fandom from the 60s and 70s and 80s felt comfortable with this that it made sense. And all from the above quoted lines from BoT.
B: The early drafts of ST:III has the BoP being of Romulan design. Hence the term BoP and the cloaking device. When the villain was changed to Klingon, the rest were likewise changed to Klingon, especially since someone remembered the Klingon/Romulan sharing noted in Enterprise Incident.
The end result being this mess we're discussing today.