Confirmed upgrade on a Latitude fare right up until arriving at the airport. Shortly after arriving a message on the AC app showed seating relocated to the Premium economy cabin. When I asked the agent what happened she said the seat I was assigned in the Business class cabin was snagged by the crew as unserviceable, the flight was full in the J cabin, so no upgrade.
You roll the dice you take your chances...
I think the seat didn't fully retract & I mentioned to the agent I was ok with that but she said once the seat is tagged as US they cannot use it.