You receive the rewards that the Clan obtained while you were a member of the Clan. If some of the rewards are locked, that means the Clan obtained the locked rewards before you joined the Clan. I.e., you did not contribute any Union Tokens to obtain these rewards. However, all subsequent rewards that you unlock while in the Clan will be available.

If you switch Clans and join one that has not yet unlocked the rewards, you will be able to claim them in the future with your new Clan.