Commutation

Everypony is born as they are. Yet, with a bit of magic, shouldn't one be allowed to bend the rules a little, to see what the respective other has? Just for one day?

If you were looking for alicorn Twilight, not here.

Through ancient rune magic, Twilight Sparkle drew together a spell that can transcend matter and exchange two ponies' racial traits. What could possibly go wrong when her beloved Rainbow Dash agrees to test it?