User blog comment:B214/Volume 22 spoilers/@comment-82.48.158.172-20161109231053/@comment-1359322-20161112233701

@Kenshin, Sona wouldn't do that. Trading servants is what most High-class Devils who don't train and solely rely on their Demonic Powers do. Those Devils don't bother training their servants and will trade them off if they consider them weak. Sona doesn't want to follow that trend that's why she even started her Auros Academy, do you think someone like that would simply "trade a piece; assuming a better piece is found".

And before people keep on going on Rias can't reincarnate Valerie and Lint due to the Longinus thing. First off, Rias can reincarnate Lint before she receives her Longinus, Incinerate Anthem is a transferable Longinus after all. Second, Valerie could've done something to Rias' Evil Pieces with her Longinus, that makes her reincarnate-able with 1 Bishop Piece.