Thanks. I modified source definitions in /etc/apt/sources.list to old-releases and was able to apt-update. Following that i ran,
wget http://archive.ubuntu.com/ubuntu/dists/disco-updates/main/dist-upgrader-all/current/disco.tar.gz
the untarred under /tmp/upgrade
and ran
python3 dist-upgrade.py
And set both disco and eoan to supported=1 in /var/lib/update-manager/meta-release. After running do-release-upgrade few times i was able to upgrade to 20.04.