Downgrades get reimbursement whenever they happen. See the main EC261 thread for more details, and the Mennens formula in the Wiki. The team that handles this are not the team you were talking to, and you have to complete the trip before you can claim. In addition to Mennens there is also a straight cash / redemption refund option, which is what BA will probably do, and on a trip of that distance it may be more generous.