I've just spent ages trying to work out how to upgrade from 18.10 to 20.04 now it's almost out. I realize I was on an end of life and would have to upgrade through another end of life and this appeared prohibitively difficult and the advice is to reinstall
I have however just achieved it by just updating my /etc/apt/sources.list from cosmic to focal - I had to remove the EOL source I was using to the standard
deb http://archive.ubuntu.com/ubuntu focal main restricted universe
deb http://archive.ubuntu.com/ubuntu focal-security main restricted universe
deb http://archive.ubuntu.com/ubuntu focal-backports main restricted universe
deb http://archive.ubuntu.com/ubuntu focal-updates main restricted universe
And then
apt update
apt upgrade
apt dist-upgrade
It gave an error about a pre-removal script for whoopsie, but I managed to sort this with
dpkg --purge --force-all whoopsie
apt install whoopsie
I realized force-all isn't great but I couldn't find another way around it, and whoopsie appears to have been able to report itself after reinstall!
and then carried on with where I was
When I try do-release upgrade, it says no new version as it seems to have performed the upgrade okay - lsb_release -a gives 20.04
Is there anything else I need to do? Have I been extremely lucky? Just wanted to put it out there in case anyone else was trying the same