Clans are just a group of friends. If you started out in one clan, and eventually move on to another, you can still have friends. I don't see the point of forcing someone to move back to their old clan, just to play the clan league.
What is the solution to players moving clans during a season? Do you want to forbid that as well? If not, the tournament page looks exactly as it does today.
Say we did implement such a rule, what stops someone from temporarily joining their old clan for a day, and switching back after the clan league begins?
Of course, a player can leave a clan and join another at any time. We cant change this steady flow.
BUT...
The thing is if you allow it now openly and with good conscience, the threshold (i hope i translated it right) is sinking to change a clan just for Clan League. The next step is that high skilled players that are not playing in the CL, because in their clan are even better players than theriselves, switch to low skilled clans in lower divisions ... Supressing the actually core and soul of a specific clan.
Don't force low-skilled clans letting chartered players play for them instead of their real representatives of a clan ... just to stay competitive.
Its like selling your clan's soul.