I have a vested interest in this question also, so I just called AA RTW again and was told quite definitely that any change (including date/time/carrier) to the first flight, will require re-issue and repricing at the then current prices and rules. Thus you would lose the 10% discount and be subject to any subsequent price increases.

Unfortunately this is the same answer that I got last year, when the more important issue was whether a 20 segment RTW would be reduced to 16 segments if you wanted to change the first flight. (Yes it would!).

It does mean that you have to pick your first flight VERY carefully!

Happy Travels.