Some posting on this thread agree with Emirates' stance.

I do but only if the aircraft substitution was unavoidable.

If, on the other hand, Emirates' motive was to sell up to 80 more Economy seats (or, indeed, to accommodate an overbooked Economy load), I hope the courts take the passenger's side. We can't have airlines downgrading passengers simply because the cabin such passengers have booked is relatively empty and that they can sell tickets in another cabin or have already sold more tickets than they have seats.

I'd have thought that if accommodating extra Y pax was the reason, Emirates' might still be "quids in" even after refunding 75% of the total price paid for a small number of F tickets.