Originally Posted by Dave Noble

Either :
(a) change to a flight that has 1st class
(b) wait

If , on the day, you end up travelling in a class lower than that which you are ticketed, then you will be entitled to EU compensation. At this time you have not yet flown in a cabin lower than that which you are ticketed for and so no EU compensation is yet due

As long as you do nothing , then your ticket will reflect the higher cabin and so compensation will become due. If you accept rebooking and a fare refund now, then you may well be due no compensation, so just keep quiet and await the downgrade

Just have the letter written so that you can post it from the airport to Emirates on the day of departure once you have the boarding pass for a lower cabin.