Originally Posted by channa

If you changed before being at the airport, make sure it shows Upgrade: Waitlisted for each one of your flights that are eligible for upgrade.

Like CO's system, there are two (2) upgrade systems at play -- advance, and airport. The difference with UA, though, is that you can see if you're on the advance upgrade waitlist with the Upgrade: Waitlisted notation in your record (you'll see each flight twice -- once confirmed in Economy, another waitlisted for First).

As for your question about checking in, UA doesn't clear at checkin. It clears immediately when space is opened up to the next person on the waitlist.