Hello Linux Geeksters. Canonical has finally fixed the ubuntu-release-upgrader package, in order to allow the users to upgrade / update from Ubuntu 12.10 Quantal Quetzal to Ubuntu 13.04 Raring Ringtail.
To my mind, this bug was intentionally placed there, to prevent the users to leave Ubuntu 12.10 for Ubuntu 13.04 Raring Ringtail, until the support period ends.
It fix the upgrade issue, just update the ubuntu-release-upgrader-gtk package, like this:
$ sudo apt-get update
$ sudo apt-get install -t quantal-updates ubuntu-release-upgrader-gtk
And now, update from Ubuntu 12.10 Quantal Quetzal to Ubuntu 13.04 Raring Ringtail:
$ sudo do-release-upgrade