No doubt BA will try to get out of it by claiming it doesn't apply in your case or some other banal attempt. However there is no get-out-of-jail card for them in this case. The regulation states:
Article 10
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.
Note that you must travel to be entitled to this compensation. If you cancel you get nothing.
In cases of flight cancellations (which isn't what is happening to you) there is an exception for flights cancelled two weeks in advance.