'my 1 cent is that there is a vanishingly small chance of any of us experiencing the extreme good fortune of our fellow poster, and getting the agency to pay for a business-class ticket; or even persuading it to fight with the airline to get a new J-class issued.

Asking a question about validity, unprompted, reveals a passenger's doubts and invites refusal, while initiating a refusal out of the blue might be difficult for the other party.

Rocking up to the airport with a business-class ticket could present a more successful strategy. The airline might well not have the appetite for a fight with the regulator. I seem to remember that at least some of the passengers had no problems when they turned up for flights with KLM after the airline screwed up its fare offers out of Helsinki to SE Asia, but those who contacted the airline were told the only solution was to cough up the fare difference, fly economy or get a refund.

Maybe six of one, half a dozen of the other. But i hope we are kept posted on the outcome