I'm nervous this is going to happen to me on a UA award from FRA-BKK on the A380.
I just called to try and get seats for the flight I've had booked using UA miles for several months. The flight isn't until November, but all of the F seats showed as full. I was unable to receive a seat and was told I'll need to do it at the airport.
If I'm likely to get downgraded, I would much rather switch to LH F if it opens up. Although, if the compensation is more than $300, I could probably manage.