How do I change the email address a customer signs in with?

The email on the profile is the customer's login ID, so changing it changes how they sign in.

1. Go to Customers → Manage Customers and open the customer.
2. On the Profile tab, replace the address in Email.
3. Click Save changes.

Tell the customer to use the new address next time they log in. Their password stays the same; if they have forgotten it, set a new one on the Change Password tab.