Blueblood.
A name that brings anger and hate for many ponies except the one with that name. The so-called nephew of Celestia is always doing something wrong and is well-known for taking advantage of his title of prince.
Mark.
A name that nopony knows. But many humans do know it. He's a regular guy that lives in a dangerous neighborhood. He's known for being one of the few good guys that live in there.

Coincidentally, they both have an accident the same day at the same hour, both of them dying in that instant.
The Reaper goes for both of them. However, being the Reaper, he can't seem to get that much fun. So, he propose them a plan: give them another chance.
What they don't know is that they are given another chance in the other one's body.
Kinda co-written with the great Lightingace.