The wiki says "If upgrade space is not available on your desired flight, you can waitlist, and it is possible the upgrade may clear before departure -- if not your name will appear on the upgrade list at the gate."

This is actually NOT true as I just found out the hard way on an ATL to NRT flight. I WAS Diamond but no longer have any status as of the end of Jan. I used the last of my GUC back in December for a trip this month (March). The ATL to NRT flight did not clear ahead of time and I was NOT added to the upgrade list on the day of the flight. After talking to several different Delta agents, including a red coat at the gate...I was told that:

If you are not CURRENTLY a Diamond on the day of the flight you will NOT be added to the upgrade wait list. Assuming this is in fact true...the wiki should probably get updated.

(What added insult to injury was that the upgrade list had only 5 people on it for 6 open seats who all cleared before boarding and right before closing the door they upgraded a non-rev....)