I've only done it once, but my wife and I had a flight with two long segments each way early this year, so I called and they were able to upgrade me immediately on two of the segments, but told me we would be waitlisted for the other two, but that they would keep our miles even if the waitlist was not productive. I watched the outbound flight and first seemed to be full, so I had given up, but the day before the flight two seats became available. I was elated, but when we got to the gate we had not been upgraded. After some shameless begging of the GA, we took our tourist seats, when she came on the plane and gave us an upgrade. We were subsequently upgraded via waitlist on the other segment of the return flight, so we sat in front on all four legs.

I subsequently found that the waitlist apparently is expunged when they hit the window where EUA kicks in, and we were just lucky that we had a very kind GA, and didn't piss her off. The suggestion I received on this forum was, if inventory appears after the EUA window starts, call immediately and see if they will do something for you.