You're right. Later LAX-FCO flights (from mid-June onwards) show the 2-4-2 configuration in Classica plus, however I doubt AZ have programmed precisely which aircraft will be reconfigured and when, so it may well be up to luck if you get a reconfigured plane or not. If you've booked Y+ and there's no Classica Plus seating you'll still get the correct miles and perhaps an upgrade into Business or some other compensation - ask at check-in.

All I can say in the end is that you have currently a 30-50% chance of getting a reconfigured 772 on any random 772-operated flight. In any case Alitalia have a system of changing aircraft routings each day depending on arrivals, so it is never sure which plane you'll get, short of it being the right type (in this case B777).