Depending on how the figures turn out, they could be better off replacing the B767s with dedicated B737 freighters and running cargo only flights on certain routes in addition to pax B737 flights. This would give them a bit more flexibility as the B737 is easier to fill, frequency could be increased and destinations combined into a single run eg Hong Kong stops in Manila on the way back to Moresby. ANG is already a B737 operator and retraining B767 pilots wouldn't be a problem.
Obviously everything depends on future demand, if the B767 is full of freight on all its routes and can still make money with mostly empty seats in the cabin, they will probably be kept on.