Once, checking in at LHR I was presenteed with a pre-written letter from EK stating that if i went on the next flight, in the same class, I would be offered a free ticket in the original class booked FOC. I also took a downgrade to Y class, where there were plenty of seats.

In the end, I was given 50'000 miles (a return upgrade from DXB-LHR) AND i was given a voucher for a free J class ticket DXB-LHR-DXB.

A separate time, from DXB-BKK I had a firm seat in J-class, and was asked if I would go in Y. I did and was given twice the mileage needed for the same upgrade, 50'000 miles in total was given for a journey that would have cost me 50'000 to upgrade...