Comment 2 for bug 1827741

This is still an issue: Just started the disco -> eoan upgrade through the GUI prompt I got, which broke on, I think, the second step (setting up software sources IIRC). A systray icon showed up saying there was a problem with the package manager (saying 'disco', which I have now configured as the default-release, is not a valid distribution).

What seems to have happened is the sources.list was updated, replacing disco with eoan sources, breaking because the default-release is no longer available. I tried updating the default-release afterwards, so my system now shows a lot of pending package updates, but I cannot retry the release upgrade since there are pending updates.

Instead, I reverted the changes to sources.list (replacing eoan with disco again), removed my default-release, which allowed starting the release upgrade process again and now actually started the upgrade.