It appears that CO codeshare flights on UA metal with CO flight numbers now get processed for UDUs at one's elite window instead of the day before travel when the flights revert to their UA numbers/designators. As noted, the window doesn't guarantee you will be upgrade that early, but all but two of my flights in the past year of UDUs have cleared within a day of the window opening, and of these two one cleared a few hours before the flight, the other didn't clear as there was no F to be had.