Shortly after the release of RHEL 6.4, the community supported clone was announced today. CentOS-6.4 is based on the upstream release EL 6.4 and includes packages from all variants. All upstream repositories have been combined into one, to make it easier for end users to work with (CentOS 6.4 Release Announcement).
Being a clone (or “derived from sources freely provided to the public by the Upstream OS Provider” as they like to call it), CentOS6.4 includes everything that RHEL6.4 does and some additional changes which are documented in the Release Notes document.
The upgrade from CentOS 6.3 should be pretty straight forward (as is with most minor releases) so I gave it a test on my CentOS 6 VM and everything worked as expected. Of course in production servers additional testing and backup is always recommended, so make sure you plan ahead.
sudo yum --enablerepo=debug update