I upgraded from Vivid => Wily => Xenial. I had switched back to upstart-sysv on Vivid but I noticed that the upgrade software reinstalls the ubuntu-standard package (and therefore systemd-sysv) on upgrade to a new release, so I had re-installed upstart-sysv after each upgrade.
I upgraded from Vivid => Wily => Xenial. I had switched back to upstart-sysv on Vivid but I noticed that the upgrade software reinstalls the ubuntu-standard package (and therefore systemd-sysv) on upgrade to a new release, so I had re-installed upstart-sysv after each upgrade.