Today where Air Travel is concerned we have to assume nothing. I always try to fly on Air Canada. this time the cost was prohibitive for the return part so I got it on ANA for a good deal less but I always get one where I can change if I so desire or need to.
You should have booked directly with the Airline. Cheapest is not always the best as tou are probably about to find out. try the Airline directly they just might do it but i doubt it. They already have your money and probably paid the outfit you got it from a commission?