Saw these FAQs in
CX website which you have find it useful:
9. How do I get a refund from the residual values of my original flight bookings?
If after you changed the time/date on your flight booking online and a partial refund of your original ticket becomes payable, the residual values will be refunded to the credit card charged.
10. How long does it take to receive the refund incurred from online booking changes?
The refund is normally processed within six weeks.