I too have been unable to change my flight - what is worse is that when I called the call centre the cost of the change had increased by A$1000. I Tried for three days which included 3 phone calls (over 5 hours) and I still can't change my flight. I need to change it due to family illness - sadly I have had to do this before with Singapore and they couldn't have been more helpful.
I think that Cathy provide great service on the plane but this will be my first and last trip with them because of the difficulty in changing tickets