Do your stock manifolds have catalytic converters on them? If they do and you replace them with ones that don't then you'll need to either remap the ECU to work without the input from the oxygen sensors before and after those catalytic converters or modify your second set of converters and install sensor bungs there and relocate the sensors there.
It's exactly what the US owners do when they replace their catted manifolds with Euro or other brand manifolds.
