Is it possible for BA to change 2nd leg of Avios QR J booking SYD-DOH-BHX to SYD-DOH-MAN?

Availability on the Manchester flight appeared overnight and that would work better for us, plus possibility of Q Suites which I have never flown in and suspect this trip might possibly be last opportunity for a while.

just tried and agent said the segments were married and she couldn’t change one segment.

is it worth a HUACA or is this genuinely not possible?