I mean this in the nicest way: you'd have to be a special kinda stupid to quit UAL more than 2 days prior to your Delta class (assuming it even materializes). If it were me I think I'd be in class at UAL 'til 5pm the day prior and have purchased a ticket to ATL at 6pm (with the local charter co. number in my phone as a backup). I'm the last to advocate burning a bridge, but UAL gets burned either way in this scenario IMO.

Congrats and good luck!