Well, you may be about to learn why they call them non-refundable tickets.

If you've already flown the outbound half of your SYD-SFO-CPH, you should call and cancel the back half. You can ask about retaining any residual value on account. I suspect the answer will be no, but you can take comfort in the fact that if you'd bought one-way tickets they would probably have been just as costly if not more so.

As for unused UA domestic tickets within the USA, you can cancel them and retain their value on account for one year and apply them to new tickets, less a hefty change fee of $200 per ticket. Typically there is not much left after the change fee is charged. Gotcha, says United.