WARNING: This exercise has been performed by experienced stunt sysadmins. Do not try this at home without professional supervision.
You will need to manually download a few packages from the repository. A full local mirror of the repository might make things a little easier, but it's not required. I use lftp or lftpget with these paths conveniently bookmarked:
Always check the updates directory first for the latest version of the rpm, or you might end up with incompatible dependencies.
Enough talking, now let's roll:
rpm -U --ignorearch kernel-184.108.40.206-170.2.24.fc10.x86_64.rpm
rpm -i --ignorearch --force glibc-2.9-3.x86_64.rpm glibc-common-2.9-3.x86_64.rpm glibc-2.9-3.i686.rpm
rpm -U --force --ignorearch rpm-* libz-* ...
rpm -i `rpm -qa | sed -ne 's/i86/x86_64.rpm/p'`
If something goes wrong and you're stuck with an unworkable, you could boot off a 64bit live CD and manually copy the files you need or chroot into your half-upgraded system.
You might need to unpack rpms with rpm2cpio. Also note a few rpm options useful in
root, justdb, rebuilddb.
If you manage to break your system in interesting ways, I'd like to know (but not necessarily help you fix it).
Please, edit this page with additional notes and tips. It's a wiki for a reason.