Only the client or staff can change the login address.

If you change the email address from the provider side then it will redirect the notifications to that address. If you change the email address the system will send all the notification to the address you change it to, so when they receive the notice to confirm an appointment or setup billing for the first time the system will prompt them to create a new password, once that is done they can simply change their email login to whatever they would like under settings->profile.

As the provider you can delete the client or staff account and recreate a new one with a new login address. If neither of those are an option just let us know in the chat the current login and the to be new login address and the reason for the change.