De-rated and reduced are different. Take the CF6-80C2 engine. Some airlines will run the newer engine on say their 767-300 fleet at nominal 68,000lbs of thrust. After engine performance has deteriorated they will de-rate them to 62,000lbs of thrust then install them on say 747-400 for further usage. De-rating is a mechanical adjustment at the fuel control unit and software change at the TMC.