User blog comment:Mnzombie/Issei's Peerage/@comment-139.168.116.203-20131206051903/@comment-1359322-20131206094142

We know a Low-class Devil can become a King by getting a promotion into High-class Devil or above. What we don't know is whether or not a Pure-blooded High-class Devil who is a King can become other Piece. So until i see one i'll assume that it's impossible. Don't know where Ruval came from.

As for the others, most of them are completely impossible. Evil Pieces can't mix. You can't become another different piece once your Piece is fixed so Akeno, Tsubasa, Koneko is impossible.

Ishibumi already confirmed that Kunou won't become Issei's servant.