My experience has been "NO CHANGE" after travel has commenced. That was even back when Ansett was in business and same experience with TG & SQ.

I gave my associate a SIN-HKG-SIN redemption seat on SQ about a month ago. As the typhoon was approaching HKG, the possibility of canceling or diverting inbound flight were great. Even when my associate arrived at the airport early, to try to catch an earlier flight, hoping to get in before the typhoon, the SQ ground agent said ABSOLUTELY NOT even though the travel hasn't even began. Instead, SQ told my associate to go to UA in the other terminal, just to get rid of him! The flight took off with a handful of empty seats - I checked.