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Greetings!
Great question. The do-release-upgrade will only upgrade an LTS version to the next LTS release by default. The 19.04 release is not an LTS version. You can read more about it here:
https://help.ubuntu.com/lts/serverguide/installing-upgrading.html.en
You may have better luck with the -d flag, just remember that you need to keep updating regularly if you intend to stay on a non-LTS release and not lose support for security updates. This may mean breaking your configurations from time to time.
Jarland
I think I figured it out - I think there’s a bug in the Ubuntu code that’s supposed to update the mirrors list that comments it out instead.
While do-release-update was paused at this message: Third party sources disabled
Some third party entries in your sources.list were disabled. You can re-enable them after the upgrade with the ‘software-properties’ tool or your package manager.
To continue please press [ENTER]
I went and checked /etc/apt/sources.list and everything except a few security entries from security.ubuntu.com. I uncommented all the references to mirrors.digitalocean.com, changed them from cosmic to disco, then hit [ENTER] at that prompt… and now the upgrade is proceeding.