In some cases EC 261/2004 passenger rights directive allows the airline to reduce the compensation by 50%. But this happens only if you were offered re-routing and the arrival time of the flight at the final destination did not exceed the scheduled arrival time of the flight originally booked:
By 2 hrs in case of flights up to 1500 km.
By 3 hrs in case of flights between 1500 – 3500 km.
By 4 hrs in case of flights over 3500 km.