That's very disappointing. I wonder if this is a one-off or only for a limited period. Regional J on a 10-hour flight is not acceptable IMO. Somewhat relatedly, this trip report in ANA J from SGN to NRT (6 hour flight), in February 2022, experienced what appears to be a one-off sub from regional 787-8 to long-haul 787-9 (the opposite scenario):


I suppose if the flight time is not changed, then ANA does not allow you to swap flights? Last year, I had my flight departure time on HND-LAX changed (maybe 20 minutes difference?) and the ANA website gave me the option to switch to any HND-LAX flight that day at no extra charge. Consequently, I made sure to choose the flight with the best J product (was originally booked in the standard J seat, then switched to a flight with the Room).