I didn't get an email either, just noticed by chance when checking BA app.
I presume they are under no obligation to inform you of change of metal, I did get an email DAYS later after noticing the plane and schedule change, I firming of the schedule change. Also, of course, BA changed our seats without notifying us either.
Disappointed too as was looking forward to first First on the 388, but maybe it's not that different to 744?