Originally Posted by CO Insider

Hi golfingboy, this is more complicated thank you might think. In the advance period, ZE class (the inventory we use for B and M instant Elite upgrades), is a more widely available inventory class. This is the case up until 24 hours before departure. At 24 hours before departure, we will prioritize expired waitlists for Regional Upgrades, Systemwide Upgrades and Mileage Upgrade Rewards ahead of all complimentary Elite upgrades, including those that would otherwise book into ZE class. This is a change we implemented only in the last couple months to ensure our Elite Upgrade Automation properly accounted for these customers with expired upgrade reward waitlists within 24 hours.