Upgrading Natty (11.04) the same should apply to other upgrades.
I use Rackspace which replaces the default apt repositories with its own mirrors. Once a Ubuntu becomes unsupported Rackspace removes the repository and installing any software results in 404 errors.
First backup the entire server as an image incase something goes wrong.
Next create a copy of the sources file for apt-get:
$ cp /etc/apt/sources.list ~/sources.list.backup
$ vi /etc/apt/sources.list
Replace all occurances of “mirrors.rackspace.com” with “old-releases.ubuntu.com” in “/etc/apt/sources.list”, then:
$ sudo apt-get update
And follow the usual upgrade path:
$ sudo apt-get install update-manager-core $ sudo do-release-upgrade
I’d recommend running
do-release-upgrade in a tmux session so it is not interrupted if you net connection dies.