Strange issue I'm having trying to make a change to a Cathay flight segment on my DONE4.

I'm currently booked on HKG to YYZ at a certain time (say 6pm), but I'd like to move to the earlier flight on the same day (2.30am). Availability showing on Expert Flyer and the AA RTW desk person could see it too, but they said they couldn't make the change as Cathay weren't confirming the seat/booking he was trying to do, or something like that.

Anyone have any experience with this or know what I could do? My current booking it leaving me with a 20 hour overnight layover that I don't want or need, so I'd really like to make the change before availability is gone (currently D5 on Expert Flyer).

Thanks!