Help from the experts.

I have a OW Award ticket using 180K miles for FC. I am on a Qantas flight BUT... the CX flight I want has opened up on the same route...was told tonight I cannot switch airlines on a OW Award ticket. I cannot believe it....I was originally told no routing changes but nothing about no carrier changes. The reason I originally took the Qantas flight was because CX was not available but I prefer CX.

Is there anything I can do to get the CX flight I want and dump the Qantas flight? Can't I just go to the airport and reissue the ticket. Geez louise, how many more restrictions do we have to tolerate.
Thanks