This has been frustrating me for years. It can't be that hard to make a fix. I experienced it again today, motivating me post. When an upgrade hasn't cleared at the 24-hour mark, we are incentivized to do OLCI at the 24-hour mark as check-in time is the factor that prioritizes upgrades among same status and fare code (and I've experienced this mattering.) But when an upgrade clears in the reservation system after you've done OLCI, now your reservation is in a weird limbo. Most importantly it becomes impossible to assign a seat. You can't do it online, and you can't do it via a reservation agent. Canceling check-in doesn't seem to help. When you try to assign a seat, it either doesn't stick or you are told it is under airport control, even when the flight is still 15 hours away. It doesn't seem like rocket science to permit getting unchecked-in and then redoing OLCI in the new cabin. I hope this gets fixed in the integration with CO - but this has been out there for many years on United, even well before UDU when were using instruments.