The way it looks, it seems that a semi-
Cosmic Retcon happened... but only for those five ponies. It wasn't their marks that got switched by the spell, it was their destinies; the mark flipping was a side effect. So you have each pony thinking they're supposed to be acting out somepony else's destiny/role in society, despite not having the ability/desire to do so. I.e. Applejack thinks she supposed to be fulfilling the role meant for Rarity, even though she sucks at fashion. In fact, that fits with my headcanon about the Cutie Marks being representative of the ponies' state of mind.
For example: AJ gets Rarity's destiny, and thinks she's somehow meant to be a fashionista. Because she believes this to be what she's meant to do, her cutie mark changes. The reason it changes to
Rarity's mark is because subconsciously, AJ knows that it's Rarity who's meant to fill that niche and knows that Rarity's mark is appropriate for somepony with that role. Because, on some level, she thinks "This is what this destiny is supposed to be", she manifests the cutie mark of the pony who was supposed to be there. They all retain some subconscious idea of who
really does what, which is why they're able to snap back once they fulfill their proper roles.
edited 25th Feb '13 9:11:42 AM by JapaneseTeeth