We have found the following guide very useful in this process. This works for both dedicated servers as well as VPS nodes:

CentOS Upgrade Guide

Notes:

1. VPS nodes do not need to be restarted after the process is complete unless you are running a PV-grub kernel (pvgrub-* in your Kernel Options inside the VPS Manager).
2. As of the time of writing, pbone.net has all of the RPMs necessary for this upgrade.
3. cPanel servers can be successfully upgraded in this fashion, however third-party RPMs may need to be removed first.