From my experience, if the aircraft suddenly becomes 3 class from 2 class and are SOLD as 3 class, then you will definitely be downgraded back to J Class at checkin - the checkin agent will pick it up in the DCS.

As for Himeno, they actually didn't swap you class at all. what is displayed on the BP may not be the actual BP. for eg. on BA they use M Class for economy class for ALL ticket type on their BP.