User blog comment:B214/Question & Answers 5/@comment-178.128.187.173-20150913135947/@comment-108.224.166.119-20150913153020

Actually that's not in regards to keeping servants when a devil has king status and their own peerage they can trade one of their servants with another king for one of that king's servants or for an evil piece. So when issei gets high class status and get his own evil pieces he can trade Rias one of his bishop pieces and one of his knight pieces for Asia and Xenovia and the other bishop piece will traded with Lady Phenix for Ravel