I was downgraded from J to Y+ as well.

I was unable to check-in online at T-24 (almost exactly) so I suspect that it had already been decided. The flight was severely overbooked and many people were downgraded in all cabins.

I also got an upgrade to F on the return journey, as did a collegue, but I noticed plenty of other people who were downgraded on the way out were still sitting in J on the way back.