1) In owncloud database change the table : oc_accounts
For the existing owncloud user in the column backend put: OC\User_LDAP\User_Proxy instead of OC\User\Database. For example:

UPDATE oc_accounts SET backend=’OCA\\User_LDAP\\User_Proxy’ WHERE user_id=’my_user’;

2) In table oc_ldap_user_mapping add the LDAP user. For example:

insert into oc_ldap_user_mapping (owncloud_name, ldap_dn, directory_uuid) values(‘my_user’,’cn=User_Name User_Surname,ou=users,dc=ris,dc=mk’,’user_uuid_from_LDAP’);

3) Delete the existing user from oc_users

delete from oc_users where uid=”my_user”;