Actually you don't need to delete the entire file. Yes the RSA key changes, you are after all re-generating it when you installed a fresh new OS. The only thing you need to remove from known_hosts file is the one line that points at your IT.

when you see something like

Offending key in /home/foo/.ssh/known_hosts:13

remove line 13 and try again, type yes at the prompt and you are once again off and running.