In Calendar Server version 5.0 and earlier, there were no domains. Therefore, the user and calendar ID's were not required to be fully qualified. That is, they did not need a domain name to be part of the ID such as email@example.com. If your uid's and calid's were not fully qualified before installing the current version of Calendar Server, your data does not have to be altered. The system assumes any unqualified uid's and calid's it encounters to belong to the default domain. If you want to implement multiple domains, however, you must migrate your LDAP and components databases to indicate which domain each user belongs to.
In addition, your data might need to be migrated in other ways. There are several migration programs. Check the migration information found in Chapter 3, Database Migration Utilities for Calendar Server 6.3.