Word so far from those who have used the MB's is that they were able to 'double dip' for now. I suspect as long as both systems are up and running, that will be the case, but once the old FP machines are done away with, I think you will only get to use t he FP+.
I just wish someone could tell me if everyone is ALWAYS going to be limited to just 3 FP+ a day and all in the same park. I HATE that feature. We never stay in one park all day, and tend to hop around a lot from park to park. This is going to limit or change our touring style greatly. In fact, I have many days of our 9 day Dec. trip with no FP+ reservations at all. The reason is that I don't want to stay in one park all day.