at T-24 you go on a special airport waitlist where your priority is supposed to be preserved.
you can check if you are on the list via mobile.united.com or via united App on android/iphone.

Priority within 24 hours
New waitlist requests for MileagePlus Upgrade Awards, Regional Premier Upgrades, and Global Premier Upgrades can be made until 24 hours prior to departure. At 24 hours from departure, waitlists will expire and automatic processing of upgrades, per the priority listed above, will continue. As customers check in, they will be placed on an airport upgrade standby list. This standby list will clear after check-in is closed, and is based on Premier status and fare class. Learn more about the upgrade standby list.