I found the following which sorts out the Radio Operator bit; it was a secret blind for the real job of radar operator.
The Royal Air Force 1939-45 - Google Books
I am not sure whether there was a direct swap RO to N brevet as then RO could certainly not navigate to the standard then require for a general navigator. I might try and see if I can contact then RO I knew although the trail is 20 years old.