I think the OP is suggesting that if he has a BOM-SYD ticketed on a DONEx and that route is then cancelled, QF would move all of the passengers onto the QF/JET codeshare BOM-SIN and then mainline QF SIN-SYD. I believe the ticket would remain valid in those circumstances, but of course not if for other reasons you have to do a reroute/reissue before the flights are taken.