In the Star Trek: The Original Series, we see Klingons for the first time. They have faces similar to that of humans. In later appearances on other Star Trek shows, their faces have ridges. Now, I'm not asking why their faces change; that is addressed in Enterprise. What I'm asking is why some changed while others do not. For example, the character of Karnas' appearance for the most part does not change from The Original Series to The Next Generation. However, the military leader Koloth has his appearance change from The Original Series to Deep Space Nine, which takes place around the same time as The Next Generation. Why is this? Both in- and out-of universe answers are appreciated.