After a lull in travel, I've been perusing FT again, and, being out of SWUs and planning intl travel, I've read with interest the thread on paid upgrades offered at the gate, via OLCI, Mr. Chicken, etc.

Wondering if there's a consensus about whether there's a greater likelihood of being offered the upgrade depending on check-in method and/or the time of check-in.

And once checked in, if no upgrade has been offered, is it fruitful to inquire at the gate?

Also curious about upgrading segments down the line. For instance, I might be doing SFO-IAD-EZE soon. If an upgrade were for sale on the IAD-EZE segment, would it be offered when I checked in at SFO?

Finally, if on UA metal but booked on a US codeshare, are these sorts of upgrades offered?

Despite a fair amount of flying the past few years, I haven't been offered any paid upgrades upon checkin, so I'm a bit baffled by the process.

Apologies if these questions have been asked and answered - search isn't working for me at the moment. RTFT replies appreciated, too. (Read The, uh, FlyerTalk Thread.

Thanks much!