Failure after FAF / 1000' would require that the MCDU minimums be reset from a 100ft alert height to a 50ft DH, (at our company), and that the Captain be fully briefed on the visibility requirement for CATIIIa and the new DH. There is a visibility change as the CATIIIB, or Airbus CATIII Dual, allows for auto-land with a reported visibility at minimums, however the flying pilot, (CAPT) does not have to actually see the runway environment. CATIIIa requires a reported visibility minimum AND the pilot must have the required runway environment in sight at the DH.
Because our flight operations manual is very strict on pilot roles on approach below 1000, our operations certificate does not allow this change if it cannot be accomplished below 1000'. It's not that it can't be done it's that many operators specifications don't allow it.
The FED's, both US and EURO, are picky about what you are allowed to do in a CAT 11 /111 environment. Going heads down at low altitude, and messing with the settings on the approach guidance during very low visibility approaches is not generally a good idea.