I recently upgraded a ticket at checking at the airport and worked out a bargain 12.5k and £61 tax to go from PE to UC. (lhr to lax)
The ticket was a economy miles ticket upgraded using an Amex voucher to premium. I was offer the upgrade for cash for ~£1400.
The upgrade was not possible for the same rate prior to 24 hours before the flight as there was no miles tickets availability. At checking the does now need to be award redemptions avalibility, just avalibility in the class.
iDo