call them up and ask WHY? they're not supposed to f*ck with your roster just to reduce overtime.
Afaik. only if they need a flight for training they can take if off you. Having said that, sometimes they will make it a training flt for a few hours and change it back to a normal line flight, but they are not required to inform you of that, so YOU need to be on the ball and step on their toes...