Calendar Server still supports operating in a non-hosted domains (that is, having a single domain) environment. For example, if you had an existing Calendar Server version 5 or earlier legacy installation, you can still operate in the single domain environment by setting the ics.conf parameter service.virtualdomain.support to "no". See also, Enabling Hosted Domains.
You will, however, still need to migrate the legacy version components database to the current version. For migration information, see the Chapter 4, Database Migration Utilities.