An airline has depreciated its new aircraft in the past over 25 years. New fuel usage and safety standards indicate that a shorter useful life is now appropriate for all of its existing aircraft. Depending on the circumstances, the airline might (a) spread the undepreciated cost of the aircraft over the remaining life of the aircraft, or (b) recognize an asset impairment loss immediately and then spread the carrying value over the remaining life of the aircraft. Under what circumstances might each of these two treatments be appropriate?