Making changes to the slapd configuration with ldapvi
Recent versions of the slapd server don't use the /etc/ldap/slapd.conf file for configuration, they store the configuration in the cn=config LDAP database.
DO NOT edit directly the files in /etc/ldap/slap.d !!! This is the backend for "cn=config".
Changes to the cn=config database can be made with any ldap tool, included ldapvi.
The procedure is the same used to make changes to normal LDAP databases: ChangeuserNameLDAP, only run ldapvi (as root) as follows:
ldapvi -Y EXTERNAL -h ldapi:/// -b 'cn=config'
(this uses the default Debian configuration: the system root user has admin permissions on cn=config)