I’m currently running 1.810.210.57-1, is there a clean method to upgrade to 5.211.65-8?
No as CentOS has no upgrade path from 5.X to 6.X. You need to reinstall. Please see http://wiki.freepbx.org/display/FD/FreePBX-Distro-1.8xx.210.58
Thanks Tony! Much appreciated.