The 777 is quieter than the A310. PIA have been saying they will put the 777 on to keep the council happy as they keep breaking the planning rules for the runway extension from many years ago which put a noise limit on night flights. The A310 is regularly late meaning it departs after the curfew and they have to get permission for the departure each time.
It will be interesting to see what happens to the loads now the A310s are going all Y as from what I understand it was the business seats they were struggling to sell.