I just did a do-release-upgrade of the OS from Ubuntu 14.04 to Ubuntu 16.04 (instructions).
Seems to have gone well.
Per the Discourse installation instructions I then performed dpkg-reconfigure -plow unattended-upgrades so that security updates are applied automatically.
I got this -
Can I assume the command worked, and this warning requires no action?
I also have this message when I do an apt-get update:
So it seems it is a backup file, since a new 50unattended-upgrades is present.
A diff of them shows
That is to say, the Ubuntu 14 version had
"${distro_id}:${distro_codename}";
as well as
"${distro_id}:${distro_codename}-security";
which is in the old and the new file.
So I deleted the backup/old file for now.
I’ve seen these before and updates have been running fine – so hang on and wish for the best 
If anything goes wrong, run sudo unattended-upgrade --debug --dry-run to watch what unattended-upgrade is thinking 
