Originally Posted by DELee

No, you don't check in again. What you do is, after at least a few minutes to a few hours after T-24, go to the app on your smartphone and select your reservation from the list shown on Reservations. When you do that, one of the options is "View Reservation". Upon selecting it, scroll down toward the end and one of the last blocks might show "Upgrade to United First / Business First / Polaris First for $xxx". If you select it, it should show the cost for the upgrade. This value can change during the T-24 hour period as the pricing and prediction algorithms weigh what it thinks the demand is versus how many seats are left.

If you've dropped an RPU or GPU on your flight(s), knowing where you are on the upgrade lists vs. how many seats are left will give you a good indication of whether the algorithms are pricing effectively - either in United's favor (lots of upsells) or in your favor (few/no upsells).

You can also see these offers online when you try to change your seat and the web page says you need to go to the Check In page because you're already checked in. On the Check In page, you may see an upgrade offer there. I've found the upgrade offers most consistently through the app, though.

David