If I've installed
unattended-upgrades what will happen when a new release comes out? Will I keep getting new packages for my installed release, or will my system upgrade to the new release?
Unattended Upgrades use the repositories of the installed version
All upgrades get downloaded from official Ubuntu repositories. The repositories are divided by release versions and are referred by their alliterating first name. For example, Ubuntu 17.10 is known as Artful Aardvark. The repository for 17.10 is called Artful.
When you setup
Unattended Upgrades for Ubuntu 17.10, the name Artful is entered in the settings. Currently there is no mechanism within
Unattended Upgrades to check for new release, and change all the repository settings. Thus,
Unattended Upgrades will continue to work as before when the new release comes out. However, it will not look for newer versions of Ubuntu or newer versions of applications in the new Bionic repository.
This is by design. When a new release comes out somethings may not work as before. This information is found in the "Release Notes" and you are supposed to read them before upgrading to a new release.
Unattended Upgrades are not for ever
When the current release reaches its end of life
Unattended Upgrades will stop working. For example, Ubuntu 17.10 will reach the end of its life sometime on July 2018. After that date, there will be more updates and upgrades for Artful. Old repositories for versions of Ubuntu that have reached their end of life are archived to old-releases location. Once that happens unattended upgrades will stop working.
Hope this helps
You will be getting updates for your current release. To get the new release (next 18.04) you will have to upgrade manually.