God I hope there are no smooth headed Klingons in this movie!!! I never saw the episode (can't even remember which show it was on) that explained the changes.
IMHO no explanation was ever needed. In the 60s the studio could not afford to make the Klingons look as they wanted to. Then during the movies and subsequent series when there was more money they could afford to make them look how they originally were supposed to look.
When I watch TOS I can suspend disbelief and imagine they look as they later were depicted.
I don't need to have all of these changes explained to me on the screen. I can use my imagination.