As I understand it, mode reversions etc aren't really relevant to alert height.
Hypothetically, if you get a mode reversion (from CAT3 DUAL to SINGLE or CAT 2, for example) below alert height you will get a triple click and should go around. However, the aircraft is still capable of completing an autoland and the autoland light will not illuminate.
The autoland warning light is there, unsurprisingly, to warn you that the aircraft is no longer capable of completing an autoland (loss of ILS signal, multiple failures etc) and will only flash in that scenario.