Originally Posted by
AAaLot
...Nevertheless, I do wonder if any one had any real experience changing the first segment for a RTW ticket 'for good reason'
Not with a RTW, but with normal AA tickets I have called before departure and been placed on standby, and I even used the 'flat tire' rule once. The key, however, is to call. Another 'good reason' I have had to make use of is the illness clause but that's covered in the rules.
If you still have time, now that you have a plan B in place, why not call AA? They could come through for you.