Hmm. I think previous episodes make it very clear that the elements of harmony are strongly tied to cutie-mark magic. Witness Discord's corruption of the Mane 6—he used anthropomorphized cutie marks to subvert their respective elements. This also explains Nightmare's earlier attempt—she was trying to do the same thing, subverting Dash's loyalty, Rarity's generosity, Fluttershy's kindness. (Applejack's and Pinkie's were not well thought out)
So, when Celestia turned the Elements of Harmony against her sister, her sister's powerful cutie mark reacted to the good intent, and imprisoned her on the moon. It wasn't Celestia herself that did it, it was a reaction between cutie mark and elements.
In other words, had it been, for example, Applejack who was rebelling against Celestia, she would not have been sent to the moon. She would have been trapped inside an apple tree, or transformed into one, or something.
Eh, speculation.
On that note: Twilight would have become a book of powerful magic (or possibly a constellation), Fluttershy would likely turn into a flock of butterflies, Pinkie would be a magical balloon, and Rarity and Rainbow I think you can guess.
Edit to add: A flock of evil butterflies and a malevolent, fourth-wall-breaking balloon are both images that amuse me.
edited 27th Mar '12 12:40:57 PM by OriDoodle
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