I think the logical argument here is that this amounts to a downgrade and that the OP would be entitled to 75% compensation (and to fly) under EC261. That seems to be the implication of the note OP received from BA. My understanding, however, is that BA has always argued that these sorts of events are de facto cancellations, not eligible for compensation as it’s more than 14 days away (but you can of course have your money back).