Ao que eu saiba, dá para fazer o upgrade pelo CD, mas é preciso usar a versão "ALTERNATE".
Veja o que está na página oficial:
Upgrading using the alternate CD/DVD
Use this method if the system being upgraded is not connected to the Internet.
1. Download and burn the alternate installation CD.
2. Insert it into your CD-ROM drive.
3. A dialog will be displayed offering you the opportunity to upgrade using that CD.
4. Follow the on-screen instructions.
If the upgrade dialog is not displayed for any reason, you may also run the following command using Alt+F2:
gksu "sh /cdrom/cdromupgrade"
Or in Kubuntu run the following command using Alt+F2:
kdesu "sh /cdrom/cdromupgrade"
Basicamente, basta inserir o CD no drive e seguir as instruções na tela, mas, repito, é preciso usar a versão "ALTERNATE".