I think that if SAS cannot deliver the sold Economy Extra product and involuntary routes the passenger to ordinary Economy in some other carrier then SAS has actually violated the contract and needs to refund some of the costs of the ticket - no matter if the LH Y would be priced the same as SK Y.

SK promotes their Economy Extra and promotes the service - it is clear that some of the passengers choose to fly because of the service and will be disappointed about the product if SAS does not deliver the promised service.

This kind of partial refund shouldn't be a problem but a standard procedure - it is then another question whether SAS could perform partial refund as points or vouchers.