The new SDK doesn't allow WP7 Silverlight or XNA projects to be created for WP8. There is a conversion utility that will attempt to turn a WP7 Silverlight app into a WP8 app but essentially it is time to start over with the new WinPRT API. There is no automatic conversion for XNA apps.
From the developers point of view Microsoft might as well have thrown away WP7 and started something completely new
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