Originally Posted by Hipplewm

So, basically it comes down to a single thing - when does it go from revenue/fare buckets control to Battlefield control.

For instance - a 1K buys a economy ticket for himself, wife and daughter to travel EWR-BCN (90 days from departure) and the fare codes are PN4, PZ3 and he applies PP and gets upgraded to Business at time of purchase. Then the fare buckets go to PN0, PZ0. The next month a 1K buys a Ticket in Prem Plus O fare class and applies PP to the reservation and a GS buy a economy ticket and applies PP. Neither are upgraded due to PN0 and PZ0. Eventually Business class sells out - but the original 1K was upgraded - do they get downgraded for the GS - HELL NO - because of fare codes at the time.
Skip to boarding and one business class passenger doesn't show up for the flight - who gets the upgrade - per the cited material it is the 1K in pre plus - BUT - we all know it will be the GS in economy

Second scenario - 26 hours before the flight it goes to PN1 and the GS calls in and has his upgrade pushed thru and the 1K gets "Jumped" neither of which follow the rules posted by UA above but they happen every day.