After the destruction of Equestria, being converted turned from life-saving to a punishment. David was converted as a punishment for a mistake he made, but, as they say, everyone deserves a second chance.
At the heart of every Conversion Bureau is 'potion', the nanotechnomagical serum that converts a human into an Equestrian. But before the Bureaus, the serum had to be created first. This, is that story.
Princess Celestia is a disembodied head in a box. Maybe she's gone insane. Or maybe, just maybe, she hasn't. Based on Chatoyance's TCB Brand New Universe Chapter 7: Mankind Triumphant
Within The Conversion Bureau Universe, Newfoal immigrants face settling new lands within Equestria. Unfortunately, not one of them knows how their new universe actually works.
An ancient manuscript written in Middle English, the only surviving artifact from long lost Earth, may hold the true reason for the emergence of Equestria.