Cool Banana is correct in that all the GE powered B763's are HGW capable providing the extra money is paid to Boeing - there are no structural differences between the aircraft.
VH-OGA/B/C were the first aircraft to get the higher weight capability because it was cheaper on older aircraft (Boeing discounts it by 5% for each year that the aircraft has been in service). From memory, I believe that the formula was US$55 per pound increase per aircraft minus 5% per year of service. As the weight increase for each aircraft was approximately 13 tonnes, it is clear that it is not cheap and thus is only paid if that extra capability can be used to increase revenue