I am going to take the Ubuntu server, ubuntu.eskimo.com, down for about 25 minutes to image it prior to starting the upgrade from Ubuntu 15.10 to Ubuntu 16.04LTS.
The upgrade will take a good part of the day. The server will be available during the upgrade but there may be a period of time where some functionality is impaired while various package dependencies are broken while part of the old packages and part of the new simultaneously exist.
After the upgrade completes, assuming it is successful, I will take the server down again for another 25 minutes or so to image it again.