Upgrade from FC1 to Centos?

I am running multiple DirectAdmin-servers on Fedora Core and on CentOS.

Can I upgrade all Fedora Core servers to CentOS 3.3?

Yes? And what problems can I expect?

Fedora Core and CentOS are different branches of development so you would not be able to upgrade as you described.
Upgrading the same branch to a newer version of the same branch should work though. For example, upgrading from FreeBSD 4.10 to 4.11