User blog comment:Collbrande-Ex-Caliburn/Hyoudou Issei's Peerage composition hopes/dreams/hypothesis/@comment-31137559-20170124200851/@comment-1359322-20170125040518

@Sky, there are no plans for the Evil Pieces and Brave Saints to be traded with each other. For starters if you trade Brave Saints to Devils, they would become Fallen Angels that would beat the purpose of the Brave Saints, to replenish the Heaven's forces and number of Angels.

And the Evil Piece system itself is the reincarnation of other species into Devils. If you change that then it wouldn't be the Evil Pieces system anymore.