also is there any forum threads from others who have tried to cancel part of a married segment?
I managed to cancel the first part of a married segment through the call centre a couple of weeks ago. It was a QFF rewards ticket (departing in Aug) in J for my wife and I, BNE/SIN/CDG with BNE/SIN in QF metal and SIN/CDG Air France, booked Oct 2021.
QF had cancelled the BNE/SIN flight months ago and rebooked us on a QF flight the next day, meaning we would miss our connection. Received emails every few months saying they were aware of the problem, and would get back to me (haha).
So eventually I rang the call centre in the middle of the night, and tried to get rebooked on a QF flight the day before our onward flight, which had Y reward seats but no J. The operator could not see these as available, and told me my only option was for her to cancel the BNE/SIN segment and for me to book the Y seats myself on the website. My heart was in my mouth but I agreed to this, but begged her to be sure not to cancel the SIN/CDG segment.
She was able to do this and said our tickets would be reissued within 24 hours.
MMB was showing the booking correctly as being just the SIN/CDG segment, but after 4 days the tickets had still not been reissued. Had to ring again, and the tickets were reissued within 5 minutes.
So palpal it is possible to do it, but it is a risk. Of course if you take the earlier flight and not the original SYD/BNE flight as booked, and you haven't cancelled it, then it is my understanding that they will cancel the second segment as a no-show.
So as I see it (and I'm only a bronze novice), you will either have to ring up and cancel the original SYD/BNE segment (and hope the call centre doesn't screw up) which allows you to take the earlier flight with better connection time, or stick with the original booking (with the missed connection risk).
Good luck!