Except for LTS Ubuntu doesn't support software updates from "cycled out" versions (like 19.04 just became and 18.10 before that)
This was the best answer I could find online it provides this very useful blog post updated in April.
My issue is:
It provides a guide to trick your computer into updating from 18.10 to 19.04, as 19.10 was the current version at the beginning of this year the update from 19.04 was still supported and as such the user could then update to 19.10 the easy conventional way, after the 18.10 -> 19.04 passage.
My "strategy" was then to 18.10 -> 19.04 this way, do the same process but for 19.04 -> 19.10, and from 19.10 to 20.04 by updating the conventional way
Just as the blog post alerts, I changed all the links for the 'disco' version in the meta-release2 file into http://old-releases.ubuntu.com/
I also deleted all the entries for the 20.04 version in the meta-release2 file too
Still - I can't do it, and it gives me the following error:
Invalid package information
After updating your package information, the essential package
'ubuntu-minimal' could not be located. This may be because you have
no official mirrors listed in your software sources, or because of
excessive load on the mirror you are using. See /etc/apt/sources.list
for the current list of configured software sources.
In the case of an overloaded mirror, you may want to try the upgrade
again later.
