If one or more user ID's need to be changed, run the csrename utility.
This utility performs the following steps:
Converts the user ID's in the Calendar Server LDAP attributes (the ones with the ics prefix). The LDAP directory is updated in place.
Renames the users in events and tasks on the Calendar Server database files. It writes the new database to a destination directory. Existing database files are not modified.
Be aware that changing even one user ID causes the whole database to be rewritten. So this is a “costly” utility to run.
For further information on the csrename utility, see Appendix D, Calendar Server Command-Line Utilities Reference.