Fans had already done so long before DS9 or ENT.
The need to explain the changes in Klingon appearance over the years. Just make the changes you want as makeup/costuming techniques and budgets improve and leave the crazy theorizing to fandom.
I actually liked that. Since their redesigned appearance first appeared in TMP fans have speculated on that for years and a couple of Star Trek novels offered up potential explanations.
I know Gene Roddenberry wanted fans just to assume they always appeared that way. But come on, Trekkies just accept something at face value without trying to theorize some in story explanation

. Perhaps it if was an alien only seen once or twice in the original series, or it was some subtle change.
But Klingons were arguably a favorite villain in the original series and the change was pretty substantial.
Even still, in canon during most of the Berman years it was left be. They generally went with the upgraded look first seen in TSFS and just left it be (obviously different Klingons differed in appearance from each other as one would expect, but they were obviously based on TSFS...and in general I considered TSFS appearance just a refined version of TMP).
It wasn't until "Trials and Tribble-ations" that any difference was even acknowledged in canon. Probably because since Klingons appeared there they couldn't just ignore it. I mean, I suppose they could have pretended they didn't look different but I don't think that would have worked. And I think they were looking for a little comic relief. Honestly I think they would have been content to leave it as that.
BUT...now that they did confirm there was a difference in appearance they then sort of backed themselves into a corner. So they finally decided to tackle it.
I did like their answer. Some novels have speculated that perhaps it was genetic, and that made the most sense. It also explained a bit how Klingons in the original series even behaved a bit differently, more ruthless, yet more prone to fear at the same time. So even some more subtle changes were offered an explanation. And it also allowed for a potential 'curing' at some point to explain how Kor, Koloth and Kang looked like TSFS-version Klingons by the time of DS9.
Discovery sort of blew that out of the water. I'm not at all fond of the Giger-Klingons in Discovery (but it's more than just continuity, I just don't care for the appearance). They did make some adjustments in season 2 and you saw some that looked a bit more familiar, like an upgrade on TSFS design. I kind of wished they threw in a couple smooth-headed Klingons in the background just to tie it to the original series and Enterprise a bit more.
Honestly, introducing the Borg is probably one of the worst.
Awe, I loved the Borg. Granted they got overused in Voyager and I preferred them more in "Q, Who?". I did love them in First Contact though. I understand some of the issues with the Borg Queen, though in retrospect it sort of makes sense. But just their appearance in FC--they were always creepy, but something about the glistening sheen, the corpse like body appearance, the camera angles and just the way they'd look at everyone made them that much creepier.