Yes I agree with Often1. Two matters:

Original equipment swap - action initiated by CX - refund of mile difference. But in CX's record you're still originally in Y.

Change of dates - action initiated by you. If no PE on that flight then reissue is proper.

I'm potentially doing something similar on a weekend trip to TYO. Hemmed in by both my and the missus' work schedules. Since the CX and KA flights are available on Avios I chose to use Avios - the max you can be charged is the YQ.