Don't take a premature offer from CX, they are playing nasty game.

I would be much more optimistic about your ability to change to other dates on J service. I did a quick check on May 7 - May 11, and for both flights HKG-SYD in these dates, the flight's showing all F9A9J9C9.... Y9B9H9... which makes me believe these are all wide open.

Traditionally, early May is a quiet time. CX would almost always say "no award availability" if you call more than 2 or 3 days prior to flight date, but once it's within the 48hour period, you should be able to change at your will. Considering the total J capacity of A333+ A346 is nearly 90 J seats, I HIGHLY DOUBT these will be all occupied by revenue pax for 6 consecutive days.

Don't take the Y-offer. Just wait around, and call back in early May. Just book the hotel for extra 3 nights, you can cancel them anytime with no penalty.

PS. If you are physically challenged, it makes even more of a nightmare to fly on Y...