Below is the recent post on the "1 clan rule".
In my view, 3. should come before 1. A player should complete all his activate games with his current clan, then the clan can remove him/her from their roster, and only then the player can be added to the roster of the new clan.
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[Official] 1 Clan Rule
Unread postby Lindax on 29 Oct 2018, 07:00
1 Clan Rule:
A player may only be a member of 1 active clan.
Clan Events:
A player can only play for 1 clan in the same clan war, clan tournament or other clan event.
Changing clans:
First have the clan you're leaving take you off their member list.
Then have the clan you're joining add you to their member list.
You can finish your ongoing clan games for the clan you're leaving.
You cannot join any clan games for your new clan until all your games for your old clan are finished.