User blog comment:B214/Volume 22 spoilers/@comment-90.219.230.127-20160730215940/@comment-9595671-20160731012529

Plus even if there was a piece trade, Bennia's a knight, not a queen. So Bennia can't act as a queen. That's probably the same reason why Kuroka had to be a Bishop on Vali's team, she was already reincarnated by two bishop pieces in the past.