Does exisiting applications work on iOS 9 or is their something special we must do or recompile it ?
Will my application that was developed for iOS 8 be compatible on iOS 9 ?
When you load the app file for the first time using the newest version of xCode (iOS9), it should run you through an automatic conversion process of your old app version into an updated correct new one.
Wether you need to recompile or not, has a lot to do with what your app does.
One of my apps, though it works fine, is now visually useless because iOS 9
switched the system font to San Francisco which screws up spacing very badly
in my app. In my case, I need to adjust for this and recompile/submit. Your app
may need no modifications at all. Test it thoroughly.