What do you mean? Belle was not a princess in the original story as you write in your post, then why would she be a princess on Once? Original Belle became a princess/Queen when she married the Beast/cursed Prince. Making her a Princess on Once would not make her character any more valid or important. Its the story they tell about her that has that effect.
Cinderella was not a princess until she married her prince. In once Ella was not a princess until she marries Prince Phillip.
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Time to hide your crazy, and start acting like a lady.