Use that form, you can do it for each flight or both at the same time after the trip has completed. In my experience they pay out in a week or so with no issues.

The reason the downgrade usually happens is they have had an equipment change to a plane that doesn't have a PE cabin. It's happened to me more than once. You might try calling IB and see if they can switch to another flight that has cabin space. Also once on board it's good to keep in mind that the flight crew are not responsible for the downgrade; I've seen people making their life hell.