I have recently installed Fedora Core 12 on my server box. It was an upgrade from Fedora Core 9 and I had to do an intermediate upgrade from 9 to 10, since direct transition from 9 to 12 is not supported. Aside from this small annoyance the process went through very smoothly. It looks like Fedora Core 12 supports an "in place" upgrade mode, similar to recent version of Ubuntu. That means that a new release of the system comes out, you don't need to create the upgrade media (DVD or a USB image), but can upgrade directly via your current session. This is very convenient in my case, because my server doesn't have an attached monitor.