I've updated from the previous LTS release to the current one some times ago. That went quite smoothly (I had to uninstall manually some packages first), but I've just remarqued something which can be a side effect: Ubuntu doesn't warn me anymore about the available updates.
Running manually the update manager suggest me to activate that in "Software sources", but I don't have an entry for Software source anywhere I could think of, and for sure not at the obvious place of "System, Administration". I've also checked that I hadn't uninstalled a relevant package, but found none apparently relevant not installed (I've update-notifier installed).
If someone could tell me how to get back the notification, I'd be interested.