For now you’ll need to manually request to unlink old, unused wallet → How to submit a wallet unlinking request
FYI, you can connect up to 4 Rewards wallet to the same custodian account. More https://support.brave.com/hc/en-us/articles/360056508071-How-many-Brave-Rewards-wallets-can-be-linked-to-my-crypto-custodian-account-