No, because the provisions relating to downgrades do not have timelines attached. Other areas, notably cancellations, do have timelines affesting payment - you are not claiming these elements. BA prefers to think the downgrade provisions apply to last minute events, but that's just their opinion untroubled by the regulation's wording. The USD100 eVoucher is a bit unusual, so you will need to do some sums how close you are to EC261's formula, given the part refund that presumably BA have now paid.