Originally Posted by DaveSFO

Here's what's worked for me.

I start with the UA search since it's pretty good now at showing a lot of partners. Look for the flights you want to upgrade as if you're trying to book an award ticket. For the most part, upgrades and award tickets come from the same inventory, so if you see it available as an award you should be able to book the flight and then upgrade it (assuming no one has snatched it up). I won't say that as an absolute because I'm sure others who know more than me would point out exceptions, but it's a pretty reliable rule.

As others have said, after finding availability I then go to the operating airline's website to book (no codeshares). Make sure you're booking an upgradeable fare (B or higher for Y to C). Once it is ticketed by the operating carrier you can request the upgrade at UA.com through the *A upgrade award page (looks like LH has a problem still). It asks for your name, ticket number, confirmation number, airline, route, date, flight number, and current class of service.

I typically get a confirmation from UA within a few minutes. If I then go back to the operating carrier's website and bring up the reservation again it shows now as business class.

Full disclosure, I've done this three times on TG, OZ, and NH. Each time was seamless but it's not like I've done it many times. The hard part was finding the award availability. To double-check, when I find availability I want on UA.com I always go and check on ANA and Aeroplan. If it's real it will show up on all 3 of the searches.