Unable to modify schema in OpenLDAP using run-time configuration cn=config
Solution 1:
If there are other olcAttributeTypes in the entry then you have to do a replace not add and list all the other attributes already in LDAP.
Try to use ldapvi for editing, it will do it automatically.
EDIT: If that doesn't work, then you'll need to stop slapd and edit files in /etc/ldap/slap.d/ manually. That certainly isn't a solution when the changes are relatively often...
Editing schema in cn=config is a new feature, you may have found a bug (possibly already fixed in newest version). Check OpenLDAP ChangeLog and try using newest version.