Does anyone know what happens to your seat assignment if you do not clear the U/G waitlist?

The situation I have is a flight tomorrow on which I had confirmed E+ seating with a pretty good seat both ways, LAX-MIA-LAX. I requested e-U/G using 500 milers and am now within 48 hours, when I have, on previous occasions, gotten an "Upgrade confirmed" message (I'm 2P.) Instead, I am now seeing my itinerary showing "Waitlisted for upgrade." In the seating, my previously confirmed E+ seat and booking class are now changed to B/C = NF and seat = N/A.

If my U/G does not clear, am I likely to find myself back in E-? If so, I may never use e-upgrades again. I'd rather be sure of a decent E+ seat.

JR