You can pass the approach ban point if you have the requisite RVR. Once past it, it doesn't matter what the RVR drops to but you must have the required visual references at your decision point in orfer to land.
This thread is about the rather unusual case where the RVR recorder lies in fog but the aerodrome is actually CAVOK. It is thus not legal to pass the approach ban point without clearing up the RVR issue with the ATCO..