reporting back. as expected for my cx549 'E" inventory came up after waiting for 2 days.
Instead of calling CX I tried to just go "change my flight" online, and to my surprise if you select the SAME DAY SAME FLIGHT under "change flight", it gives option to rebook to "E" class without any change penalty.
What's better I got some Tax Refund

And it gets better - my e-ticket was reissued, so I have 1 year from reissue day (today) instead of 5 days ago to use the ticket.

I have a feeling if I called CX they will demand the change fee ($2k HKD?) so I am glad their online system is such a garbage that you can exploit the system.