Always ask for an upgrade, especially if you're above base level on their rewards scheme. I got heaps of Emirates upgrades to BC doing this. Also, I once had my seat double booked on an a380, was a great economy with good leg room. If this happens, and the plane is at full capacity, don't fight for your seat. Arrived at seat, taken, check boarding passes, double booked. I told the lady to keep the seat, called a flight attendant over to sort it out, BC upgrade right in front of her!