"Normally" Cat 1 reversion is due to an equipment failure and should be decided by 1000, re-briefed exec etc. There is no point in having briefed and set up for Cat3 to revert to Cat1 just because the weather conditions permit. Autolands need to be practiced. Of course if its Cat3 and below a 1000 there is a failure, normally go around, but to revert to Cat 1 makes no sense. If the met is below Cat 1 then you can't revert, simple. At DH you must have the required visual reference to land.
"If, after passing 1 000 ft above the aerodrome, the reported RVR/VIS falls below the applicable minimum, the approach may be continued to DA/H or MDA/H.
The approach may be continued below DA/H or MDA/H and the landing may be completed provided that the visual reference adequate for the type of approach operation and for the intended runway is established at the DA/H or MDA/H and is maintained"