upgrade problem from ubuntu server 12.04 to 12.10

I have installed ubuntu server 12.04 from a LiveCD adn it works fine with me. Now I need to upgrade to ubuntu 13.10. It is mentioned that I cant upgrade directly so I have to upgrade to 12.10, then 13.02, then 13.10 (http://ift.tt/1gqNqlZ). In the process of upgrading to 12.10, I am able to "apt-get update, but "do-release-upgrade" yields an error:

Checking for a new Ubuntu release Err Upgrade tool signature
404 Not Found [IP: 91.189.88.161 80]
Err Upgrade tool
404 Not Found [IP: 91.189.88.161 80]
Fetched 0 B in 0s (0 B/s)
WARNING:root:file 'quantal.tar.gz.gpg' missing Failed to fetch Fetching the upgrade failed. There may be a network problem.

I changed the sources.list and was able again to do the update but yet the same error is appearing.
How could I solve this problem?

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upgrade problem from ubuntu server 12.04 to 12.10