Business class award ticket booked via UA. Aircraft changed to 77W (from 787) and now I'm not able to make seat selections (auto assigned middle and aisle..) Is this a known issue during aircraft swap? The message I'm receiving says I have to wait until check in to select seats..

Update: Found another thread mentioning it needs to be selected through the app. Bolded/underlined it on the Wiki as I missed it the first three times..