It still hasn't been announced whether members will be receiving a discount for their tickets. All I've seen so far is this statement from Viagogo which looks promising.
From Eric Baker, CEO, Viagogo
Everyone at viagogo is deeply shocked and saddened at the news that Michael Jackson has passed away. Our thoughts are with his family, friends and fans. All Michael Jackson tickets purchased through viagogo will be refunded directly to fans. No forms, no fuss, just refunded. We will be in touch with everyone in due course.