Calendar Server 7: Coexistence with Calendar Server 6
By Joesciallo-Oracle on Oct 14, 2009
As the Calendar Server 7 docs state:
(you can)...set up Calendar Server 7 (CalDAV Server) in an existing Calendar Server 6 deployment, where calendar users exist on both the Calendar Server 7 and Calendar Server 6 environments. In such a deployment, you enable both freebusy lookup and iCalendar Transport-Independent Interoperability Protocol (iTIP) invitation between the two Calendar Server deployments. That is, users should have the capability to check freebusy information of users on any server, and the capability to invite any user. Invitations to users on a different server would be delivered by using iTIP.
Now, Andreas has some more info on this setup, here.
(Photo Credit: muhammad younas)