After a telepathy experiment goes awry, Pinkie becomes Twilight and Twilight becomes Pinkie Pie. Now they can't swap back.
But that's okay, because they're eerily good at it.
You don't need to read the prequel to get this. It's good by itself.
For her birthday, Pinkie gives Twilight a book on how to be Pinkie. At first dubious, Twilight becomes fascinated.
After a telepathy experiment goes awry, Pinkie Pie becomes Twilight and Twilight becomes Pinkie.