With earlier versions of Calendar Server, Calendar Express (the old user interface) was automatically installed and enabled. You could not disable it, even if you did not use the interface. If you are upgrading to Calendar Server 6.3, the upgrade process adds service.http.ui.enable="y" to the ics.conf file. This keeps the old UI enabled if you want to use it, no further action is necessary.
To disable Calendar Express, set the value of service.http.ui.enable to "n" in the ics.conf file.