Changes the user name and, if a security profile exists, assigns
    the security profile associated with the old user name to the new
    user name. This might be used, for example, when a person's name
    changes (as through marriage), and, therefore, his or her account
    on the operating system is changed accordingly. The new-username
    must not currently exist in the database.

    When the ALTER USER command is issued, the existing user name is
    removed from the database and replaced with the new-username. If
    SECURITY CHECKING is INTERNAL, then subsequent SHOW PROTECTION
    statements will display the new name for the user, and all GRANT
    and REVOKE statements will require the new-username. The new-
    username is not visible to other sessions until the transaction
    containing the ALTER USER command is committed.

    See the RENAME for further discussion.