My fails but finally successful with fedupFirst, on my personal computer, I used dnf (a new, much faster successor-to-be of yum; I recommend it to try it out) to update all packages to the latest F19 version and then ran fedup utility with "--network 20 --nogpgcheck" options. Turning-off GPG checking was the fastest way how to work-around some signature issues. fedup reported some un-available repositories, so I just disabled those for the sake of upgrade.
After downloading packages and installing new fedup drucut I restarted computer and it proceeded without problems. And it still works fine.
So, to sum it up, on my personal computer it went very well, only some un-available repositories and GPG issues were encountered during the whole upgrade, but those were easily worked-around by using appropriate options.
What to do if fedup is not an optionThe day after I proceeded with the same steps on my working computer, which is Lenovo T510. I saw the same issues with un-available repositories but not GPG issues any more. So I disabled the repositories and restarted computer after all packages had been downloaded, the same as I did on my personal comuputer.
When fedup dracut started I saw kernel panic after 7s (similar to RHBZ #873459, but I can't reproduce anymore, so I'll try it again on one of my virtual machines -- well quite paradox that I got kind of a heisenbug in a process of getting Fedora Heisenbug) so basically in a very early state. What is more, the same happened when I tried to boot with the newest kernel-3.12.x. But I was still able to boot with older kernel-3.11.x. So I wasn't screwed totally, but I was not able to finish fedup upgrade even repeatedly.
My solution was to go on with another way of upgrading -- upgrading using yum. I put dnf aside, since I wasn't sure if it is able to deal with upgrade (now I regret that I didn't try it) and using distribution-synchronisation option it went very well. After some hours (yeah, I have too many packages) I ran restorecon on whole root to be sure SELinux will work fine and rebooted. A bit surprised, I saw a new Fedora Heisenbug successfully booted and didn't noticed any issues.
Now I'm happy I could welcome the New Year with the bleeding-edge features that are served by Fedora 20 Heisenbug and I would like to wish all (not only) Fedora maintainers to have many solved issues, more interesting challenges and a lot satisfied users.
Guys, thank you all for your contribution in 2013!