Comment 4 for bug 1602066

The update-manager gui will also behave differently if your HWE stack is no longer supported. If your system is out of date (e.g. there are some packages in -updates not installed) then you will be presented with a dialog regarding "New and important security and hardware support update". The dialog will have an install button which will upgrade your HWE stack to the next LTS. After installing the updates and rebooting you can run hwe-support-status as described in comment #3 and should not see anything unsupported.

update-manager will recommend release upgrades over installing a new HWE stack so if all your packages are up to date you will not see the "New and important security..." dialog. One way to force the dialog is to set /etc/update-manager/release-upgrades "Prompt=never".