Thanks for the answers! I called the elite desk this afternoon just in case. In my particular situation there were no changes, nor was anything oversold.

The (very helpful!) agent explained that the computer didn't see that the original flight coupon had been used, probably because of a mess at the airport. If I understood correctly, the reservation system wasn't aware that I had taken the outbound flight, so the record was out of sync. She was able to fix it without any more work on my part.

I guess that this situation could cancel a combined CO/UA trip as in jtstoney's post, especially on the way home. (Was that the case, or did you get the message before the whole trip? Did you get reaccommodated?)

So if you found this thread searching for the message, here are the possibilities so far:

• Result of a change you made yourself, such as a postpurchase upgrade.
• Schedule change. New flights/times should show up. Wait for an email with the new schedule.
• Oversold flight. If you call in advance to reschedule voluntarily, you may get an ETC.
• Electronic equivalent of a mislifted coupon. No actual changes to be found. Call before flying. (And hope for a good agent?)