> How did you manage to install fedora without "fedora-release-xxxxxx.rpm"

Didn't.  The fedora-release-xxxxxx is installed.  The idea is to upgrade fedora-release to the new distro, then upgrade from there.  I.e., if you're upgrading from fc7 to fc8, you already have the "fedora-release" package for fc7 installed.  Use rpm -Uhv http://somemirror/pub/fedora/linux/updates/8/i386/fedora-release-8-5.noarch.rpm to upgrade "fedora-release" (the .repo files) to fc8.  then do a yum upgrade.

Of course, if you have other .repos (e.g. atrpms, livna) use rpm -Uhv to upgrade them to fc8 too, before doing "yum upgrade".  Its probably a good idea to do this first:

to see what you've got.