Just completed the update from IMM30B with FA02 radios to IMM76K with FC04/FC05 radios. Here's what I did:
1. Reverted back to ICL53F radios via CWM and a recent nandroid backup and rebooted (wiping cache and Dalvik cache prior)
2. Flashed EK02 radios (zip file via CWM) and rebooted
3. Checked that all went fine - it did

4. Flashed the new OTA IMM76K-from-ICL53F zip file (with the recovery folder inside the ZIP archive removed to keep CWM) and rebooted (wiping cache and Dalvik cache prior)
5. After successul reboot I'm running IMM76K with the new FC04/FC05 radios
However, I lost root and OTA RootKeeper was not able to restore it (I made SU backup prior to the whole procedure). So I booted into CWM recovery again and flashed the modified SU for 4.0.4 that is floating around and rebooted. This restored root for me.
So far I have no problems. It's actually quite amazing that I was able to do that without running into major issues. I just rooted my Gnex last week (long-time webOS user and first time root ever!) as I lost hope that Verizon would push out an update any time soon.