I have a multi-city itinerary HND-HKG-ORD-xHKG-HND booked online (on the CX JP website) in R class. I called CX HK and tried changing my HND-HKG sector from 543 (morning flight) to 549 (afternoon flight) but agent informed that the flight was W0 R0 E0 (and seeing that I had knowledge on fare codes, 'assured' me that it was also D0 I0...). (I have no access to inventory tools so I'd also appreciate if any veterans here can help check with that...)

Funny thing is, I tried to make a dummy booking HND-HKG-HND for 549 that day and I could reach the payment page for R class. And most importantly, the flight is on Jan 2nd - can a flight be 'zeroed out' in so many of these fare classes (not V/O/L.. etc) at this much time in advance? The agent also commented that it was weird and said I might try calling again later to see if they 'open up' again.

Will it make a difference if I call CX JP directly? Or is it the case that HND-USA-HND would have different inventory compared with HND-HKG-HND? Should I wait for the moment - has anyone here saw inventory 're-open' on their flights? Or should I ask for ex-NRT and see if they have inventory in I class, which probably won't be that much of a difference in fare?

Many thanks in advance I guess these are the troubles of doing ex-XXX runs...

You've gotten the correct answer. Married segments inventory control. You need to do dummy booking on HND-HKG-ORD-HKG-HND in R class.