Not to mention: if you're not at least using configuration management by now, you're doing it wrong. Rebuilding a server should be a completely automatic process.
In fairness, RHEL/CentOS did not support in-place upgrades between major releases until RHEL/CentOS 7 (just released). Prior to 7, it was recommend to always do a fresh install.
How so? In what way does Ubuntu not support this?
Not to mention: if you're not at least using configuration management by now, you're doing it wrong. Rebuilding a server should be a completely automatic process.