Players can join a Clan part way through an event in order to obtain these extra Event Challenges.
If you are removed from your Clan, or choose to Leave a Clan during the event, you will no longer be able to participate in the Clan Event Challenges. After the 24 hour cooldown you can then join a different Clan. This will give you access to any active Clan Event Challenges, and any Clan Group Challenges will be updated to the current total for your new Clan.
If the Clan has already completed and claimed a Clan Group Challenge, the new member will not be able to claim that reward. Players will not gain double rewards by Clan hopping.