Do all of them work on iPad Pro or do they have to be changed to support it? Is there a way to have a separate iPad Pro app from your iPad app? (To do things like have iPad Pro exclusive features?)
My guess is that all iPad apps will work on iPad Pro, and there will surely be some apps that update to take advantage of the larger display, which will be up to the developers. Thanks to adaptive UI and size classes, there should be plenty of apps that will be ready for the iPad Pro.