MOのテックサポートからクラン解体についてやっと返事が来たので引用します。

The clan maintenance functionality of Magic is very limited right now. There is no way to assign another clan member as captain.
What should happen is that the clan captain appoint another member as captain.
The only thing I can do is delete the clan and have the new captain request the clan again then have everybody join the clan again.

Jeff
Wizards of the Coast
Software Support
Magic Online Store

　要は、キャプテンを任命できないのでクランを削除するしかないそうです。

　というわけで、旧「Magical Planet」の皆さん、その方向でいいですか？

　何もなければ、その方向で問い合わせてみますね。