Share my experience.
I bought this promotional fare as TPE-NRT-LAX-SJC, SJC-LAX-KIX-TPE on aa.com stock 001.
I tried HUCA for about 4 times to try to change the return route. No luck.
So I kind of gave up. So I tried to call to change the date only for LAX-SJC. It went through quickly and charged me around USD$6 for tax, which I believe it is for stopover.
For the return, I really want to do SFO-HND-TSA instead of SJC-LAX-KIX-TPE.
I tried HUCA for about 7 times. I only called once a day with different time, morning, afternoon, night, after midnight. As of yesterday, I successfully changed my ticket that way I want.
If you speak Chinese, you can forget about the Chinese reservation desk. They are instructed not to help in anyway.
Thank you.