Not sure if this helps, about 2 years ago, I was flying DXB-SYD EK419. Upon boarding the flight I noticed 2 vacant seats in J. I paid to upgrade the DXB-BKK leg
Anyhow; I complained later on for the treatment I received at BKK EK lounge.
In their reply, EK CS perhaps misunderstood my explanation and said:'
I do regret to learn that you were not able to upgrade to Business class as there were no seats available at that time. May I take this opportunity to inform you that bookings for Skywards Upgrade Rewards to be made into “A” / “C” sub-class only in the First and Business class respectively. The check-in counter staff has definitely no intention to appear unhelpful but he was merely restricted by the non-availability in “C” sub-class.
So to answer your question yyzlhr, I think the answer is no; (might as well sleep that hour
) And who knows, you might get op'd up because you got in late?!