When doing sudo apt update I had
Ign:1 http://archive.ubuntu.com/ubuntu cosmic InRelease
Ign:2 http://archive.ubuntu.com/ubuntu cosmic-updates InRelease
Ign:3 http://archive.ubuntu.com/ubuntu cosmic-backports InRelease
Hit:4 https://deb.nodesource.com/node_12.x cosmic InRelease
Err:5 http://archive.ubuntu.com/ubuntu cosmic Release
404 Not Found [IP: 91.189.88.152 80]
Err:6 http://archive.ubuntu.com/ubuntu cosmic-updates Release
404 Not Found [IP: 91.189.88.152 80]
Err:7 http://archive.ubuntu.com/ubuntu cosmic-backports Release
404 Not Found [IP: 91.189.88.152 80]
Ign:8 http://security.ubuntu.com/ubuntu cosmic-security InRelease
Err:9 http://security.ubuntu.com/ubuntu cosmic-security Release
404 Not Found [IP: 91.189.91.39 80]
Reading package lists... Done
The solution was to point my sources.list to the http://old-releases.ubuntu.com/ PPA. Keep your original source.list as we will need it in it's original version a little later…
Would become:
``` deb http://old-releases.ubuntu.com/ubuntu cosmic main restricted
deb http://old-releases.ubuntu.com/ubuntu cosmic-updates main restricted
deb http://old-releases.ubuntu.com/ubuntu cosmic universe
deb http://old-releases.ubuntu.com/ubuntu cosmic-updates universe
deb http://old-releases.ubuntu.com/ubuntu cosmic multiverse
deb http://old-releases.ubuntu.com/ubuntu cosmic-updates multiverse
deb http://old-releases.ubuntu.com/ubuntu cosmic-backports main restricted universe multiverse
deb http://old-releases.ubuntu.com/ubuntu cosmic-security main restricted
deb http://old-releases.ubuntu.com/ubuntu cosmic-security universe
deb http://old-releases.ubuntu.com/ubuntu cosmic-security multiverse
```
You can now safely run sudo apt update && sudo apt upgrade to update your packages in Comic.
Then recover your original sources.list file and now replace every "cosmic" term with "disco":
``` deb http://archive.ubuntu.com/ubuntu disco main restricted
deb http://archive.ubuntu.com/ubuntu disco-updates main restricted
deb http://archive.ubuntu.com/ubuntu disco universe
deb http://archive.ubuntu.com/ubuntu disco-updates universe
deb http://archive.ubuntu.com/ubuntu disco multiverse
deb http://archive.ubuntu.com/ubuntu disco-updates multiverse
deb http://archive.ubuntu.com/ubuntu disco-backports main restricted universe multiverse
deb http://security.ubuntu.com/ubuntu disco-security main restricted
deb http://security.ubuntu.com/ubuntu disco-security universe
deb http://security.ubuntu.com/ubuntu disco-security multiverse
```
Then run sudo apt update && sudo apt upgrade again.
Now things are back in place, you can safely run sudo do-release-upgrade and upgrade to Eoan normally.
By vinyll on April 9, 2020