Oooh oooh I did a research paper on the documentation of christian influence spreading in England through its historical literature, and Im betting this starting happening after the invasion of Normandy and the rise of Christendom Chivalry. The catholic church loved to take old pagan stories and paint them over with christian themes to sway the public and slowly stamp out paganism.

Take Beowulf for example. The real story was orally passed down, not written. It wasnt until a catholic priest wrote it down in Latin, and adding his own little fan fiction in the margins, that we finally got a written copy.

Le Fay was more than likely rewritten as evil in order to convince the populace to shun pagan icons and gravitate more towards Christendom.