I have searched through all the OLCI and upgrade threads, and can't find this specific topic.

I have an ORD-IAD-LHR flight on 13 Feb that departs at 6:00 a.m. Flight is on UA metal, but a BMI codeshare. Because of this, I know that I cannot request an upgrade using a SWU until the day of travel.

I have a flight from SEA-ORD on 12 Feb (not on UA). Is it possible to request this upgrade then in SEA? If not, should I try if I can find someone at the UA counter when I arrive at ORD? Or do I have to wait until I get to IAD?

If I want to upgrade, should I do OLCI or wait to check in at the airport? Not sure how it works with a codeshare.