Paid uprades can be proposed for any fares, including award tickets. And is the cost higher for lower fare buckets ? Indeed it should be logical but, if your read this whole thread, the different data points we have collected so far show no relationship between the booked fare, the distance flown and the price of the proposed upgrade. And the last datapoint given by CDGcommuter is a good example : very cheap upgrade and booked originally in A fare, which is the cheapest Premium Eco fare. OK BOS-CDG is a short route but this is still a very very interesting price for Premium Eco to Business ^