I can only cite my personal experience, FWIW.

I bought my first AONEX ticket in 2001 for 2002 travel ex-ARN (good price at the time). AA RTW set it up and it was originally ticketed through a travel agent. It was an AONE4. After ticketing, the price ex-ARN was increased.

I later converted it to an AONE5 (added South America) and one of the recongnized expert AA RTW agents did the conversion/re-issue. I was charged what the AONE5 price ex-ARN would have been at the time I originally bought the ticket.

Not quite the same situation and, recall, that at that time, one could get the other country's price when the ticket was purchased in the United States, but maybe it is helpful.