Trying to see if anyone knows the "unspoken" policies when it comes to rebookings (and involuntary downgrades).

So I booked JAL Business from LAX-KIX-HND a flight T-180 out. Both segments were in business. At T-120 days i get a notification that my flight was cancelled. I was automatically rebooked for 24 hours later.

However, the second segment KIX-HND was rebooked as economy when business class seats are available for sale. Are they literally allowed to do that?

And what are my options if I want to keep the original date? I know LAX-KIX is cancelled, but they are still selling LAX-HND (final location). Is this a possibility?