Então, encontrei uma solução alternativa:
- Abra / var / lib / update-manager / meta-release e encontre strings:
Dist: precise Name: Precise Pangolin Version: 12.04 LTS Date: Thu, 26 Apr 2012 12:04:00 UTC Supported: 1 Description: This is the 12.04 LTS release Release-File: http://archive.ubuntu.com/ubuntu/dists/precise/Release ReleaseNotes: http://archive.ubuntu.com/ubuntu/dists/precise/main/dist-upgrader-all/current/ReleaseAnnouncement ReleaseNotesHtml: http://archive.ubuntu.com/ubuntu/dists/precise/main/dist-upgrader-all/current/ReleaseAnnouncement.html UpgradeTool: http://archive.ubuntu.com/ubuntu/dists/precise/main/dist-upgrader-all/current/precise.tar.gz UpgradeToolSignature: http://archive.ubuntu.com/ubuntu/dists/precise/main/dist-upgrader-all/current/precise.tar.gz.gpg
- Fazer o download dos arquivos UpgradeTool e UpgradeToolSignature
- Carregue-os em qualquer outro servidor da Web e modifique as sequências correspondentes (acho que até o arquivo: /// funcionará)
- sudo do-release-upgrade
Trabalhou para mim, espero que ajude você também. Boa sorte!