I am trying to enroll a new Member who also has a Customer account. When I enter his information, I get notification that his email is already in use and cannot be used again on his new Member account.
Each email address can only be used on 1 Life account. If this is the case, change the email address on the Customer account since that will no longer be necessary. Then you'll have freed up the new Member's email address for use on the new account.