Even if you follow the steps in here, you might encounter a pair of problems after upgrading Fedora from 17 to 18 version.

1st problem: Yum database corruption

The first problem arises as soon as you try to use yum in the just updated system. You'll get a message like:

> sudo yum update
(...)
DB_VERSION_MISMATCH:  Database environment version mismatch
(...)
Error: rpmdb open failed

To fix that, you only need to remove some files created by the former Fedora 17's yum:

> sudo rm -i /var/lib/rpm/__db.00*

2nd problem: PolicyKit cannot start

The second problem is that X won't start. I don't know exactly which error messages got me to the right Google search (see References) but the messages you can find in /var/log/messages might be:

Sep 22 23:40:29 grumpy systemd[1]: Starting Authorization Manager...
Sep 22 23:40:29 grumpy systemd[1]: polkit.service: main process exited, code=exited, status=1/FAILURE
Sep 22 23:40:29 grumpy systemd[1]: Failed to start Authorization Manager.
Sep 22 23:40:29 grumpy systemd[1]: Unit polkit.service entered failed state.

In /var/log/Xorg.0.log I could see something went wrong but no many clues, just an error about the number of displays out-numbering the number of the devices or the like (sorry, I cannot find a copy of the error message).

To fix that, reinstalling polkit and systemd would suffice:

> sudo yum reinstall polkit systemd

I hope this protip was useful for you!

References

• Fix issues with polkit aka Authorization Manager after reboot