Before you run csrename, you must first:
Create an input mapping file (-m option) for the users you want to convert.
Create any new users in the LDAP directory server, if necessary.
Stop Calendar Server.
To run csrename, you must log in as icsuser (or as the Calendar Server runtime user ID specified during configuration). If you run csrename as superuser (root), you might need to reset the permissions for the new database files. To modify the LDAP directory server attributes, you must also have administrative rights for that directory.
If your Calendar Server installation has a front-end/back-end server configuration, you must run csrename on each back-end server.