Pilaf The Defiler wrote:The Glenmoril Witches have a slightly different character model from the normal Hagravens. They have white hair and feathers and slightly more elaborate costumes. I don't recall if the ones on Solstheim were any stronger than the ones in Skyrim, but they're tougher than normal Hagravens. I also have no idea what changed over the last 200 years that transformed them and made them hostile. They used to be three reclusive but nice old human ladies. Same with the Glenmoril witches in previous games and lore. The whole hagraven pact must have been a relatively recent historical development we don't have all the details for yet. Forsworn lore claims that Hagravens have existed since at least the earliest human settlements in Tamriel, though. It's just that they weren't always synonymous with the Glenmoril.
I think that is to distinguish them from "normal" Hagraven's. Hircine's touch, perhaps? They're the only witches that are devoted to him that I know of. The white feathers and hair might have indicated age. . . though I'm sure the black-feathered ones have been around a while for the most part. Red eyes . . . maybe they're albino Hagravens.
I think the whole "old witches becoming Hagravens" thing is only new lore to us, introduced by Skyrim. I think it is an evolution of the idea from Bloodmoon, where one of the three witches turned into a raven. Lore-wise, as stated by the Forsworn source, witches and Forsworn shamans have becoming hags for quite a while. The "spirits of the Reach," if I recall correctly. The power and long-lifespan (perhaps immortality at the cost of their looks

) that comes with being a Hagraven is probably very honorable to witches and Reachmen. Their hostility probably just comes from the fact that they're hideous, bitter-old witches that don't like travelers intruding on their nest.
Hagraven isn't synonymous with Glenmoril, only the ones in Skyrim. Melisande, as you said was a nice old lady; so were Isobel, Ettiene, and Fallaise before they underwent the transformation, probably for the same reason that any witch would. They were Imperial women, they weren't going to live forever. . and a longer life plus ancient magic? Perhaps humanity is a small price to pay for that to some.
Tbh, it is an interesting concept. . I like that they add little touches in the various games and DLC that deal with this. It is sort of similar to vampirism or lycanthropy, with some obvious differences. (girls' only, the magic, the reverence they get from witches/covens and people of the Reach for whatever reason. . . )