I have often made use of changing the return on a round trip international ticket with minimal or zero fare difference (since the reprices usually use historical fare).

I have a ticket I'd like to make a change to that was priced as a one-way (virtually certain) but consists of a layover. It's GRU-MIA (six weeks layover), then MIA-MEX. If I'm correct and it is priced as a one-way (e.g. GRU - MEX w permitted stopover in MIA), if I want to make a change to the second leg (MIA-MEX), do I risk a reprice of the entire ticket or do you think I can rely on historical fare rules? Does it matter whether I wait to in fact fly GRU-MIA?​​​​​​