When easyJet ordered its NG's they were only 3A. After we'd received about about 17 NG's (up to JT), Mr Boeing introduced the roll-out channel which gave them 3B capability. As Boeing is not able to retrofit this mod on the earlier NG's, we operate all our NG's to CAT3A. As the flight deck is pretty much identical, it is not safe to have your crews operating both versions as the actions required in case of an engine failure on approach are fundamentally different. i.e 3A go-around/3B land.