Ah Reversion. Why would you 'revert' to CAT2 from CAT3? An airfield might change to CAT2 if it allowed increased traffic I guess, but they're unlikely to change their contract with you once they've told you and you've accepted a CAT3 approach.
In a 757 we have 3 autopilots, and at some point around 1500' they all come online, self test and you're expecting to see LAND3, Flare and Rollout armed.
Alternatives are LAND2 or MANLAND which means there's a 'problem' with one or two (although they might still be working)
In the event that you get LAND2 - the plane can still autoland it's just that we don't quite trust it as much - so there is an airplane specific limitation of 50'/200m - this is what we call reversion minima. Although it might sound like it, it's not CAT2 or 3A - you're still flying the same approach, you just have to rebug 50' on the radalt.
In the event that you get MANLAND - then obviously you're doing the landing.