I recently called the gold line to ask about this. I wanted to upgrade a coworker after a long week away. I was told that the request has to be made at the same time as the request for my upgrade. Mine had already cleared.

What she did was look for open seats and checked her fare class. Once she confirmed that there was an open seat, she downgraded me, submitted the request, and got us both in.

I usually travel alone, so this was new to me as well. I'm surprised it worked, the flight was oversold as it was.