Comment # 4 on bug 899506 from Michael Schr�der

I guess you must not remove the old /etc/HOSTNAME. I think the error happens
when rpm tries to rename the old /etc/HOSTNAME file to /etc/HOSTNAME.rpmxxx. It
tries to do this because at the time the file fates are decided the preinstall
script is not run.

So when you then mess with the system in the preinstall script, rpm's
expectations are no longer valid.

So I would remove the "rm /etc/HOSTNAME" line.