You have been given excellent advice here from the experts.
if I may add my experience with a BA issued ticket 2 years ago on an Aone5. I had a mle -hkg-sin- Mel segment that was made impossible by a schedule change by CX. After working with the BA gold desk and not finding a routing that did not add 3 more days to the trip I asked the agent for a routing not allowed by the rules the agent asked a supervisor. I requested a routing on qr male-doh-Mel and in a day or so it was approved, even though I backtracked through doh again in first class on a qr A380! On another rtw an aone6, I had a routing syd-doh-cai. Egypt banned flights from Doha so I was routed syd- doh-amm-cai. Three weeks before departure there was a schedule change making connections impossible so I asked an received a change with an illegal routing: syd-sin-lhr-cai. So many things are possible with a flight cancellation I’d you can get a supervisor to ok it. It would be possible. But the flight cancellation has to occur first.
i hope this helps.
Good luck!