I am try to upgrade to 12.04 from 11.10 using alternative CD. I run this command
sudo gksu "sh /media/iso/cdromupgrade"
after mounted iso as CD. Its gives this error "An upgrade from 'oneric' to 'precise'"
Check my image carefully. There is a typo in codename oneiric is correct not oneric.
and correct this error.
Now everything is working fine.