User blog comment:B214/Volume 22 spoilers/@comment-82.48.158.172-20161109231053/@comment-30289101-20161113001001

In Sona's case, obviously the traded piece would have to want to be traded. Sona absolutely would not do it just for power sake, so that was an obvious prerequisuit assumption to a trade taking place at all. My comment was more directed at the assertion that a tactition in general would never trade. Sona, because she actually cares about her peerage is actually more likely to accept a trade for an empty piece if the member actually wanted to leave, because their happiness would be more important to her than their lost value. However, I doubt even Tsubasa would want to leave her to join Issei, so sort of irrelavent.