Moreover, applications for Windows Phone 7.x will automatically run on Windows Phone 8 with little or no changes to the code of the app. Apps coded on Windows Phone 8 will conversely run on Windows Phone 7.x as well, so long as they don’t use native coding (apps coded with native C++ won’t run on Windows Phone 7.x since they use different APIs).
http://www.wpcentral.com/microsoft-officially-announces-wind...