I have been told that it's not uncommon on that route and it's happened twice to me (although it was an "almost"). I have been told that it's for two reasons. First, there are a lot of sheiks and other high-level people in the UAE and UK travel the route and they will usually get priority. Second, I am told there are a lot of full-fares on that route, and therefore there are a lot of cancellations and shell reservations. Consequently, they oversell the plane significantly higher than on most routes. It's just the luck or lack of luck of the draw. But, as I said, I had two close calls. Both were DXB-LHR. The first was resolved ahead of time, but the second was resolved at the gate. The GA had reserved a spot in the lower class for me and said unequivocally that I would receive compensation, though he didn't say how much. Like christep said, others have said they have received cash for the fare difference and that it's calculated as the difference between J and W on the sector. My guess would be about $500 or so.