There has been some disruption to on the HND-JFK route since Weds 18 July due to 'aircraft maintenance', including a downsize from 777-300 four class to 777-200 three class when I flew on Thursday 19 July.
In that context, JAL is very good at customer service and at process.
As an example, they called me 2 days before my flight and offered 30000 JPY (c.300 USD) to switch to an ANA flight departing for Narita 1hr earlier and, when checking in at JFK, they had printed copies of the revised cabin layout to assist seat selection.
For me, there was no upside to the switch, nor materiality for a ticket costing more than 8000 USD.
300USD would have comfortably covered the cost of a taxi from NRT to Tokyo and not much else.
I think that any offer of compensation would have already been mentioned if it fell within the scope of any JAL ex-gratia customer scheme/policy.
As per CPH-Flyer, customer relations might be an avenue to pursue (but with little to no expectation of success).
If you do, please come back and update us on the outcome.