Thanks Mwenenzi. My original route booked was London – Perth (change at Doha) – Adelaide – Brisbane – LA – Dallas – Washington DC – London. The trip began in November 2025. I have got as far as Washingon DC; only the Washingon DC – London leg remains unused from the original plan. I wanted to add some additional flights within the USA for this summer (June – August) but did not know what they would be at the time. Now I do know, so I've asked:
1. To cancel the booked final leg home, Washingon DC – London
2. To add four new flights (Washingon DC – Montreal, Boston – Dallas FW, Dallas FW – Bozeman, Seattle – Dallas FW)
3. To add a different flight home, Dallas FW – London
I'm convinced this is within the rules. I think if there was an issue with the rules, they'd have told me what it was, but the only message I'm getting (eight phone calls now, total about 5–6 hours being passed around) is a flat "we can't do that" with no explanation. I genuinely think my request simply hasn't reached someone who can handle it.

As of this morning, because of my insistence on being put in contact with the people who my enquiry is permanently being referred to (variously called "the back office team" and sometimes "the pricing team"), I was told that "the pricing team will email you with details". That would be a real breakthrough if it happens.