Warning:
You can't change award tickets. You'll have to cancel them, incurring a 50% penalty on the miles used. You can cancel ticket up to 24h before a flight. Book wisely!
ETA: Apparently, this applied to bookings made before September 2012. As at October 2015, changes are permitted and all cash and miles are refunded on cancellations up to 30 minutes before departure. In both cases, only a 20 Euro admin fee is charged.