Oh… That whole legend thing, about the statue on Canterlot Castle grounds. I’ll believe that when I see it.
Memory serves, though, it’s not about swapping a personality to its opposite, it’s shifting it into a form that causes mayhem. It would only work in direct opposites if somepony’s personality was governed almost solely by a harmonious attribute. Something like… that animal caregiver, Fluttershy, going from meek and kind to outrageous and hateful.