So new page is back up and looks like "change my booking" is back and it looks like it would work. I've tried with my own booking and like many other First world airlines, FINALLY we have ability to change flight. For my restricted V fare, it can search for other flights and clearly indicates it cost HK$xxx dollar to change, and also it shows how much it cost to upgrade to new fair bucket (but within same class of service). I didn't proceed to final page but looks to be a promising start finally.

Howeve for restricted fares, my understanding is it cost change fee for entire transaction, not per segment. For example by calling CX Reservation, if you change both outbound/return it only cost say $600 to change, but online looks like you can only do 1 by 1, meaning if you change both, it might charge you double fees