As has been noted in other threads, you should not over-react. That incident is my incident, and I promise to you that every word I wrote in that thread is the truth, despite efforts of certain CO-flag-wavers to discredit me. That said, it is also true that this doesn't happen to everyone, and it doesn't happen all the time. So you need not assume it will happen to you.

If you are confirmed in (I), then this means you're traveling on an award, right? And, you have a seat?

If you were on the wait-list to upgrade, and if it's true (as I believe it is) that position on wait-list at T-24 is determined by check-in time, then, I'd suggest being very aggressive to get on that list as soon as possible.

However, if you have a confirmed and paid for seat, then that seat should generally be yours, unless there is a down-guage and they have to start picking people for a downgrade, which does happen, but is clearly not a daily event.

Some might suggest calling to check that all is OK, if you want to spend more time on the phone. I would be hesitant to do that, because if you allow another agent to touch your record, you also introduce risk of it going "pear shaped" in the process.

Since you are starting ahead of IAD, I am guessing that you are probably naturally checking in 4-6 hours before the IAD flight. With that, you should be fine. Just get to the airport 45 minutes earlier than normal to give them time to work through shares snafus. That's my advise. But, if you get screwed, I'm not responsible. ;-)