Although vanity has disguised itself in many different forms--from Neolithic's Medusa to the modern Ugly Duckling--the social importance of being beautiful has not been eroded. Beauty and the Beast is but another rehashment.
"When Beauty is the Beast" is a little clever of saying that the concept of Beauty is the story's ultimate villain.
Disney's Beauty and the Beast focused on the beast: a selfish, barbaric animal that learns to love. In Beaumont's version, the prince was a kind gentleman cursed by a wicked fairy. It is Belle who undergoes a transformation in that she learns to see past appearances. Recall Shrek.