I can't answer your specific question, but can report that since about 3 days ago of the 6 bookings I have in MMB both short and long haul, business and economy, some many months away are showing "your seat will be allocated 3 days before the flight" whereas previously, I has able to see the seat plan and have the option of buying seat reservations.

I am Blue and travelling with kids. I have previously complained about not being allocated seats with my kids as BA is supposed to do.

I suspect that BA.com is simply doing some website updating. I shouldn't worry. I'd be very surprised if BA knew about an unscheduled change of aircraft that far in advance.