Interesting question. As is mentioned above, now on the bus we have to go around except if sufficient visual references are available.
On the 737 a downgrade from LAND 3 to LAND 2 meant that we could accept CAT IIIa 50ft RA (CAT IIIa) instead of no DH (CAT IIIb). We always bugged all relevant minina, which is 50ft RA, CAT I minimum (which showed up as a triangle on the altitude band but without callout unless activated by rotating the minima switch). So in a case of downgrade simply press the minima switch, continue to 50ft and decide as usual. In case of a NO AUTOLAND message, re-bug to CAT I and decide there. Of course that was only possible above AH which was 200ft and therefore usually coinciding with the CAT I minimum.