The magic “upgradeany” switch
I finally decided to move the backoffice systems from Fedora to CentOS. Not because it is better, but because I am bored with the insane update cycles that Fedora has. CentOS offers me five years of fixes, so I do not have to do the yearly upgrade shuffle just to keep my systems up.
I like doing upgrade installations, because it keeps the setup information and I do not have to start from scratch. When doing Fedora -> Fedora updates, this is no problem. The installer (I run this through PXE) finds the existing system and offers updating.
CentOS does not. Googling around also does not really help. However, in the depths of the Fedora Wiki, there is a list of additional options for the installer. And as CentOS is based on RedHat Enterprise Linux, which in turn is based on Fedora, you can add upgradeany to the installer command and voila: CentOS upgrades a Fedora installation. Took me just half an hour to find.
centos fedora linux upgradeany