I recently had this, and so I called up the Silver BAEC line and a helpful agent removed and re-added by frequent flyer number in each of my existing bookings.

As she updated each one, I was able to log onto MMB and select my seats for free.

This was indeed a real treat having never had Silver status before, it felt good to snag Club World mini cabins and even 1A+1B on an upcoming transatlantic trip