Cheers, that sorted it.

Unfortunately I had all kinds of other issues – like rpm removing libraries that other system components where relying on (most notably libnss which basically stops anything useful from working). I did manage to get a bit further but in the end I think this is my cue to just ditch Centos and use something else.

Thanks for your help anyway – and if anyone else is considering doing this, definitely consider just using the Fedora CD to do the upgrade for you.