It depends on your company ops manual.
Our states:-
Equipment deterioration.
If an Aircraft or Airport systems failure necessitates a reversion to a degraded approach Catagory then the approach may not commence ( ie descend below 1000' AAL ) or continue, if already below 1000' AAL unless the reported RVR readings are at or above the minima for the degraded approach Catagory.
So...... Downgrade from Land 3 to Land 2 then you MUST Go Around if the RVR's are not at or above Cat 3A.
That's our policy anyway.
