This is one of those questions: it's doable, but if you need to ask, probably not by you;-)
It might also be possible to force Anaconda to do the upgrade.
There is some probability > 0 that you will be left with
orphaned and
unsupported packages, depending on what you have installed.
Thanks John.
Basically if you have done updates to the system.. you will have 'orphaned' packages. Fedora Core 5 and 6 kernel, glibc, (or other things) have higher rev's than what is in CentOS-5. The best fix is to do a backup of /etc and other config files, backup your /home /var/I_do_stuff_here partition and install a fresh CentOS-5
A couple of suggestions for doing the fresh install and minimizing some of the pain. 1. tar /etc to someplace safe. While you cannot just restore the files, you can use them as a reference. There are pieces of /etc/passwd, /etc/shadow, /etc/samba/smbpasswd that you will want to copy to the new system to preserve users passwords. Also your /etc/samba/smb.conf file will probably just copy across. (assuming you are running samba). Other data such as that from /etc/resolv.conf will come in handy when fixing the network configurations. 2. Do rpm -qa > /some/safe/file.txt so you have a list of the packages installed on your FC6 machine. 3. I am assuming your root partition is separate from user data in places like /home. If not, you will need to make sure you have a backup restore plan or the users get testy. You will want a backup anyway just in case something goes haywire. If /home is in a separate partition, you will be able to tell the installer to leave it alone.
If you are really fortunate, your root partition is on a separate physical drive. Then you can stick a new drive in and do the install. The old drive becomes a good fallback.
Just my 2 cents.
Bob Styma