Thats what I would have expected since I have booked this ticket directly on the BA website. I had reserved 10G on the 777 which does not exist on the 787 so I would have been forced to change my seat. The only email I received was after I changed my seat selection myself.
To be honest, this was not a huge deal since I actually prefer the 787 but for a group, this could have caused problems. I guess vigilance is the only solution :-)