My flight with Cyprus airways has changed to Flyhellas and I havent been told. Why?

I booked a holiday to Cyprus on Olympic Holidays. I am due to fly on 17th August with Cyprus Airways. I checked my departure time and found I am due to fly with FlyHellas, a chartered airline. I still fly at the same time, however Manchester Airport doesn't have my flight number registered, its only registered at Heathrow. Why has this changed? Also FlyHellas are cheaper which means I have paid extra. What can I do?