In the event that you order any supplemental services along with your ticket (e.g. a better seat, higher travel class, entertainment package, luggage services, etc.) and duly pay for them, the airline is then obligated to provide these services to you. If the airline does not provide them and no reason is given, you may be entitled to a refund of your payment for the unused services.
In the event that this is specifically the downgrading of your travel class and concerns a flight subject to EU Regulation EC 261/2004, you are entitled to the return of the amount of the price of the airplane ticket depending on the distance of the flight according to the following rules: