There most likely will be a cancellation charge. The 10K bonus stipulates that you must complete your return trip. Partial flights will not qualify. Each time you do a change there will be a change fee of at least $100. So if you feel that you are not going to use that ticket, refund it. But you will not be getting the full refund back.

The new CX bonus is only 5K miles only when you complete your roundtrip. The fare might be lower but you are going to lose a lot more than if you are going to hang on to your existing ticket.

Better call CX. They have veto over what we say because they are the ones who issued the ticket. And call the internet fares desk - NOT regular reservations.