You're confusing two things. It's not suggested to go from, say, CentOS 5 to 6 in place. However, going from 6.4 to 6.5 is expected. Edited to add: per the rel

You're confusing two things. It's not suggested to go from, say, CentOS 5 to 6 in place. However, going from 6.4 to 6.5 is expected. Edited to add: per the release notes, emphasis mine: "Similar to the practice of the upstream vendor, there is no supported path to 'upgrade' an installation of a prior major CentOS release (presently CentOS 5) to a new major release."