If you want to do a release upgrade with an unsupported version of ubuntu, it seems to be quite straight forward, but nonetheless, you run into a couple of difficulties, because the sources are moved from archive.ubuntu.com to old-releases.ubuntu.com which results in not finding these if you want to do a release-upgrade.

What did the trick for me was to manually set the IP-Adress of the server old-releases.ubuntu.com in the /etc/hosts file for archive.ubuntu.com and security.ubuntu.com.

All other suggestions like changing the url in the /etc/apt/sources.list and modifying the /etc/update-manager/meta-release file didn't work, because i.e. of wrong checksums and in the end it always tried to contact archive.ubuntu.com for an updated version of the packages.