The Windows Phone 8.1 SDK is not your typical SDK update. Don't be fooled by the minor version update, this is a massive release. It includes not just one, but two app models; so-called legacy Silverlight 8.1 and Windows Phone 8.1. Silverlight 8.1 is useful when upgrading your existing WP8 app to take advantage of the new 8.1 features. Your WP8 app still compiles in Silverlight 8.1, upgrading is a relative painless effort.