Booking partner rewards often throws up issues such as this one. The ffp (BAEC) can't do a lot because they are limited to award space offered by the partner (without an over-and-above effort to liaise with the partner airline). The operating carrier doesn't care as it's not their ticket (yet).

Considering that the new itinerary is still legal and in many respects an improvement on the original, it's hard to argue anything other than an OP wants to make a voluntary change, and therefore is subject to the cancellation/change policy of the ticket. Can't hurt to ask BA for a full refund though...