I was kind of reading canon characterization as a given, since if they don't have their core symbolism intact then they're not really themselves anymore. I couldn't ever imagine a scenario where Enjolras as an ultra would feel like anything other than character assassination, either, though I suppose that it's possible in the hands of a really good author something could be done.

As for seeing him as a brunette... I could, but only in specific circumstances. In a lot of reincarnation scenarios I could see him having markedly different features, to fit in with the society that needs him at that time. (I love reincarnation fic, I really do, but I always get so caught up on details that are Not Important like "but that name doesn't fit here" and "what features would fit best but how do you still get across who is who" and it all spirals out of control from there.) But just changing his features willy-nilly, no, not a good idea. Especially since he's one of the few characters we actually have canon features for.