Per EUR261/2004 pax is entitled to 30% reimbursement from the base fare of his ticket:

Upgrading and downgrading

1. If an operating air carrier places a passenger in a class higher than that for which the ticket was purchased, it may not request any supplementary payment.

2. If an operating air carrier places a passenger in a class lower than that for which the ticket was purchased, it shall within seven days, by the means provided for in Article 7(3), reimburse

(a) 30 % of the price of the ticket for all flights of 1500 kilometres or less, or

(b) 50 % of the price of the ticket for all intra-Community flights of more than 1500 kilometres, except flights between the European territory of the Member States and the French overseas departments, and for all other flights between 1500 and 3500 kilometres, or

(c) 75 % of the price of the ticket for all flights not falling under (a) or (b), including flights between the European territory of the Member States and the French overseas departments.

However the client has to ask for it.No airline will ever volunteer to refund.