I ran do-release-upgrade from a terminal. The process eventually failed because of "too many errors". The root cause appears to be this:
Removing systemd-shim (9-1bzr4ubuntu1) ...
Removing 'diversion of /usr/share/dbus-1/system-services/org.freedesktop.systemd1.service to /usr/share/dbus-1/system-services/org.freedesktop.systemd1.service.systemd by systemd-shim'
dpkg-divert: error: rename involves overwriting '/usr/share/dbus-1/system-services/org.freedesktop.systemd1.service' with
different file '/usr/share/dbus-1/system-services/org.freedesktop.systemd1.service.systemd', not allowed
I appear to have worked around this by manually removing /usr/share/dbus-1/system-services/org.freedesktop.systemd1.service, purging systemd-shim via `dpkg -P` and then doing `apt-get install -f`, which completed without errors. After that, another `apt-get dist-upgrade` was apparently enough to complete the upgrade.
Could you please add the log files found in /var/log/