I got a small, arm-based device. I edited
/etc/apt/sources.list to have it pointing on
old-releases.ubuntu.com instead of
archive.ubuntu.com . At this point everything's fine, the repositories are found.
When I launch a distribution upgrade, nevertheless (from 08.04 to 08.10), apt looks for the new repositories at
archive.ubuntu.com, and they are no longer online.
I suppose it should look for them at
old-releases.ubuntu.com instead, but I'm not able to tweak in order to have it done.