I just did at checking Go to Plus upgrades on IAD > CPH and Return. Go was full and my plus ups released 2 additional seats (assuming in both directions) - talking to the gate agent at IAD, they didn't upgrade anyone for free(at least at IAD), so if there were no Go seats available and you were stand-by, you were out of luck...

I didn't bid on the outbound flights - just asked and paid at the gate. On the return, I bid $300 and was denied, then turned around and paid 1500DKK (~$250)... Go figure...

Jim