I've noticed that when I'm not upgraded at my DM window, the upgrade often is confirmed in the middle of the night, about 3 am in ATL, starting a few days before the flight. IIRC I had one clear in this time period in the early morning of my flight. This makes me think that 2-3am is when some inventory management system does a sweep and the algorithm automatically releases some upgrade inventory according to a formula that might be automatically changed in this timeframe.