I don't think legacy apps will auto-scale. I think they'll take up the same number of pixels, but the extra screen real estate will have a widget bar, showing things like the weather or stock prices.
I highly doubt it. That would be very un-Apple. I think there are a lot of legacy apps that will scale automatically to utilize the extra height, or there will just be a little black bar above/below the legacy app, so the app takes up the same amount of vertical pixels that it used to.