A few years ago, I encountered that many passengers were downgraded to J to Y. Fortunately(?), my booking was economy.
On the last day of Japanese holiday season, CX scheduled to use 747 for CX508 from HKG to NRT(most late departure to NRT on the day).
The flight was sold as C-Y two classes.
Saddly, last miniutes ship change occured.
Inflight Service Manager kept apologiing downgraded people on the plane.
She finally come to greet to me, and told terrible story.

I had connection BKK-HKG-NRT.
I was not able to get boarding pass of HKG-NRT flight.
Checkin agent at BKK told me that she was unable to issue boarding pass for HKG-NRT because of system could not assign a seat and seat assignment were required to be done at HKG.
Wow.