Yep, it's another op-up thread

(For those who aren't aware, an 'op-up' is an operational upgrade - moving an economy passenger to the business class cabin because economy is oversold. These are particularly interesting to DL elite leisure travelers, because we often can't afford or justify the M fares necessary to use a systemwide upgrade, and op-ups are the only other way we have a remote chance at sitting up front.)

I've been hanging around the DL forum for quite some time and am familiar with the past threads on op-ups, but I'm still hazy on two aspects and wanted to raise them for discussion:

1. Does checkin method or time make a difference for an op-up?
   
   I can imagine that if I were a gate agent, I'd be more likely to process an upgrade in advance for someone who had already checked in to register theri intent to show up at the airport. But, in the inverse, I could also see a world where it's easier to upgrade someone who doesn't have a boarding pass yet.
   
   I've personally been op-uped in both situations (once, five minutes before departure at the gate, and once, before I even checked in), so the answer to this question may legitimately be that it doesn't matter at all and that upgrades are processed in priority order no matter what.
   
   If someone happened to ask a gate agent this question during the "walk a mile" event in ATL, it would be really interesting to hear an authoritative answer.
   
   
2. What are the best indicators in advance that a flight is oversold and that an op-up is a possibility?
   
   I've found that neither the availability numbers nor the seat maps are reliable indicators. Take a flight I have coming up, for example - ExpertFlyer shows J0 D0 S0 I0 Y7 B5 M5 H1 Q1 K1 L1 U0 T0, but there are 8 open seats in J and ~20 open seats in Y on the seat map. Who knows which is more accurate and what the true loads on the flight are - two days ago, there was no availability in M or below, and business class availability has fluctuated from 8 to 2 to 0 in a matter of days without the seatmap changing.
   
   I've noticed in the past that seat maps often fill up quickly right before T-24 when checkin opens... but I don't understand why a passenger wouldn't get a seat assignment at time of booking if there are open non-preferred seats on the seat map. Are there other methods of booking tickets that don't result in advance seat assignments?