Those are the restricted extras packages, they are not essential to upgrade and no longer available (it seems) for Natty.
Easiest thing to do is comment them out from your sources.list. Open it up like this
Code:
gksudo gedit /etc/apt/sources.list
search for "extras" and put a # at the beginning of that line or lines.
The run sudo apt-get update and then continue the upgrade as normal. You will need to add the extras later as you get nearer to the current release, but cross that bridge when you get to it.
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