How to Mass Assign/revoke roles to existing supplier portal users

Summary:

We have a requirement where we have created a custom role for supplier users to allow them to create work confirmations in the Supplier Portal, and we plan to remove the existing role currently assigned to them.

However, we observed that:

The custom role cannot be automatically assigned to existing supplier users.

The existing role cannot be removed in bulk for multiple supplier users.

Since we have a large number of active supplier users, we are looking for a way to mass assign the new custom role and mass remove the existing role for these users. Could you please advise on the available options or recommended approach to perform this activity in bulk?