FWIW, on Feb 5th, the EU passed some draft amendments which clarifies the compensation for involuntary Downgrades. I'm not sure of the process to finally enact them however.
The main thing is that compensation is based on "Flight Price", not "Ticket Price".
Draft changes start from page 11:
amending Regulation (EC) No 261/2004 establishing common rules ...
The full lists of amendments to this draft in the passing of this are here in this 2.1 MByte word document
Starts from page 341:
Wednesday, 5 February 2014 - Strasbourg - texts (Part 3)
"Ticket Price" has been removed from article 10 to be replaced by (newly defined) "Flight Price":
(t) ‘flight price’ means the value obtained by multiplying the ticket price by the ratio between the distance of the flight and the total distance of the journey(s) covered by the ticket; where a ticket price is not known, the value of any refund shall be the supplement paid for a premium seat on the flight
Article 10
Upgrading and downgrading
- 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.
- 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
- 30 % of the flight price for all flights of 1 500 kilometres or less, or
- 50 % of the flight pricefor all intra-Community flights of more than 1 500 kilometres, except flights
between the European territory of the Member States and the French overseas departments, and for all other flights between 1 500 and 3 500 kilometres, or
- 75 % of the flight price for all flights not falling under (a) or (b), including flights between the European territory of the Member States and the French overseas departments.
"Ticket Price" has been specifically defined as well:
(s) ‘ticket price’ means the full price paid for a ticket, including the air fare and all applicable taxes, charges, surcharges and fees paid for all optional and non-optional services included in the ticket,such as all costs for the check-in, the provision of the tickets and the issuing of the boarding cards and for the carrying a minimum amount of luggage, including an item of hand luggage, an item of checked‑in luggage and essential items, as well as all costs related to payment, such as charges for paying by credit card; .the ticket price published in advance always reflects the final ticket price to be paid.