Same thing has happened to me although my trip is in June.
BA called me to let me know the changes, I kind of went along with it, but I just got an email from them (3 weeks later) saying:
We recently contacted you to let you know that we had to cancel your upcoming flight with us (booking reference XXXXXX). This is because we had to make some adjustments to our schedule as a result of delayed aircraft deliveries from the manufacturer.
We're sorry we are not able to carry you to your destination as planned and understand the inconvenience this will have caused.
We have already adjusted your booking and you can view it in Manage My Booking.
If you're not happy with these changes, you can either claim a refund, or you can rebook to the next available British Airways operated flight that departs either three days before or 14 days after your original flight, free of charge.........
Which made me think, I wonder if I can still get it cheaper? so now I'm doing as golfmad has said and seeing which flight I ideally want and then I'll call BA.