Last time I had an economy leg on a business award I monitored it every day. It did become available and I called in to make the change to business. Although there is no $90 change fee as you are already on the flight, be aware that the YQ fees are recalculated and I needed to pay an extra $20 as that was the amount that had changed between when I had first booked the award ticket and the time I had the leg changed to business.

I am currently monitoring a YYZ-YVR leg for a December SYD flight. I have checked every day since I booked the trip last January. So far no second seat has been released that I am aware of - my husband got the first one. I'll keep watching!