This is very simple. When airlines rebook - they always use the highest booking class for the cabin you're traveling in. That doesn't mean they pay the highest price. The logic is simple - every airline uses their fare classes differently - some airlines have Premium Economy, Basic Economy, etc., so it's hard to match one booking class to another. Thus, it's a standard practice to use the highest booking class, which is usually uniform across airlines, for Economy - Y. That doesn't mean that airlines pay each other the full price of a Y ticket. Also, because of arrangements, availability, it may be that only Y has availability and no other booking classes. Instead of checking every single available booking class - the logic is that if there is availability, Y will show it. For other cabins, similarly. Yes, the mileage posts as the new booking class but that doesn't mean that if the one-way rate for the flight is 2,000 Euros, that's what the airline pays. They have agreements between each other for displaced passengers and the rates are defined. The new booking class has nothing to do with it - it's a matter of convenience to find availability.