If the flight is overbooked in Y or J, CX will not return A or D inventory into A or D but only into F or J. So when the agent releases your A4 JFK-HKG it could come back as A4 JFK-YVR and F4 YVR-HKG for example (or F4 on both legs). Then it would be up to another department in CX to convert the F4 to A4, and their job is to maximize yield including minimizing overbooking expense. They may be thrilled to have those 4 extra seats to use and won't ever give them back as A inventory. The booking agents have no way to see this except by seeing how heavily booked the rest of the flight is. You'll have to decide how much risk you are willing to take (my guess is that the next flight with 4 A seats available is months away). BTW CX is not unique in this, most airlines do not return released seats back to their original inventory but into whatever category the yield management software decides is now optimal.