It’s been a few years since the Equestrian War was started. King Sombra has spread his dominion, shrinking the available land to farm food with. In desperation, Celestia orders the Edible Ponies Act. This act mandates that all ponies who aren’t fighting in the war nor have an occupation contributing to the war must volunteer to become food for the soldiers in the war.
 
Now, obviously she wouldn’t let ponies die like that, so she created a reformation spell and made it mandatory for all who sign up to have the spell used on them. It would allow them to reform after the pony had been digested. This way, the army would never run out of food and everypony would be contributing.