Upgrade email user on a domain/subscription to a customer/subscriber/admin

[warren1025]

How do I upgrade an email user on a given domain/subscription to a customer/subscriber/admin for that domain/subscription? When I try to create the customer/subscriber/admin, I am not allowed to use that person's email address because it is already in use. If I delete that email address, I still am not allowed to use that email address because it is reported that the email user is already in use although it is no longer visible as an email address of that domain. The given email address was a result of migrating the given domain from 9.x into 10.x, and the domain admin status of the email account holder was lost in the migration.