Complementary upgrades as early as 96 hours before departure
UA seems to be making a case for false advertising with this verbiage.
A late evening RDU - EWR flight. First is pretty much empty (guess, no one cares to pay extra for 1hr in CRJ). Up until the last second before boarding begins, the upgrades are only offered for cash, and not a single person from modest list is upgraded.
Each single person ended up upgraded in a single go just before boarding was announced.