Until a ticket is started, only the place that issued the ticket can touch it. In this case, that's BA, in others, it could be a travel agent (though I've had some agents that just auto accept any change made by the airline without checking with the client if that change it acceptable).

On one of my LONE4's years ago, I had 16 sectors booked when QF cancelled the 2nd last flight (NRT-MEL). I was rerouted via SYD and given paper tickets (this was as the full transition to 100% etickets was about to happen). Later on, during the trip, riots started in BKK and many governments and airlines were issuing travel warnings and wavers. I was meant to travel LHR-BKK-SIN-KIX. Due to the travel warnings, I skipped the BKK stop and the agent rebooked direct to SIN. As half the trip had been completed, it was rebooked as an eticket and I was told to return the remaining coupons to the agents when I got home.
BA still insisted on taking the coupons from the old ticket for the BA legs between mainland EU and Asia.

I would benefit from some suggestions on how to proceed with the following issue that I reported few posts above about a month ago:

At this time, I am debating whether I should wait until I fly the first segment of the DONE5, and then call AA RTW desk to hopefully have them fix the issues as they promised, or whether I should go ahead and contact BA ASAP and before flying the first segment to have them fix the issues. My worry about contacting BA is the fact that, if they interact with an unhelpful AA agent, I might end up loosing the option to have the ANC flights re-instated in my itinerary. And, I also worry whether the RTW ticket will be priced anew even if the changes are needed due to AA flight cancellations... On the other hand, waiting after the first segment is flown means waiting until late November to find out if indeed AA will keep to its initial promise...

So, I wonder your thoughts, whether it would be best to wait until November, right after the first segment and take it from there or rather attempt at fixing the itinerary now with BA...

Thanks..!

Provided AA has left notes in the PNR about what they told you, I don't think there would be an issue talking to BA.

Regarding the ANC flight schedules, it now seems that LAX/DFW/PHX-ANC are all during the summer months and no AA flights outside that time. 3 flights per day, then nothing. However, LAX-ANC is operating now which is not summertime. What's happening?

It's not summer, but it IS cruise season. Lots of carriers serve ANC during the tourist/cruise period (roughly May 15 - Sept. 15.)