If this was a revenue reservation in F (ie not using points) and no alternative flight was available on the same day offering the cabin booked I agree with the OP. I don’t think refunding the difference in fare between F and J is unreasonable at all. It is no difference if I was booked to fly in J and the cabin was overbooked and I was downgraded to premium economy – i’d expect the difference in fare as a MINIMUM. Aircraft substitutions are not something outside the airlines control so they should accept responsibility.
I agree also with DoorsToManual – the Middle Eastern airlines have a great product but are useless with post service recovery. Two of my own experiences – one with QR following two aircraft subsitutions to an inferior product and the other to Etihad about their cancellation without notice of a ‘mistake fare’ went with an auto reply and nothing further, not even an apology.
However…. expecting a reply from the Senior VP within 24 hours ust ain’t gonna happen….with any airline.