Comment 23 for bug 924079

@Tommy do-release-upgrade downloads an upgrade file which does some things that can't be done in the actual packages (install upgrade prerequisites, purging packages, fixing proprietary xorg drivers etc.) You can see the DistUpgrade.cfg script for Precise at http://archive.ubuntu.com/ubuntu/dists/precise/main/dist-upgrader-all/current/precise.tar.gz

If you want to complete the upgrade with do-release-upgrade rather than dist-upgrade, just make sure apt is upgraded first:

sed 's/oneiric/precise/g' -i /etc/apt/sources.list
apt-get update
apt-get install apt python-apt
sed 's/precise/oneiric/g' -i /etc/apt/sources.list
do-release-upgrade