FYI, Ubuntu 16.x now uses systemd by default in place of upstart.
Permanent switch back to upstart
Install the upstart-sysv package, which will remove ubuntu-standard and systemd-sysv (but should not remove anything else -- if it does, yell!), and run sudo update-initramfs -u. After that, grub's "Advanced options" menu will have a corresponding "Ubuntu, with Linux ... (systemd)" entry where you can do an one-time boot with systemd.
If you want to switch back to systemd, install the systemd-sysv and ubuntu-standard packages.