You will have to keep two XAPs under the same app GUID: the original one for Windows Phone 7 and the new one optimized for Windows Phone 8.
To create a Windows Phone 8 app, either create a new project targeting Windows Phone OS 8.0, with the latest version of the SDK (Version 8.0), or update an existing Windows Phone OS 7.1 project.
This means that you can create, submit, and maintain different versions of your app for Windows Phone 7 and Windows Phone 8.
One of the key advantages of multiple XAPs per app GUID is that you can associate different metadata, such as long description, with each XAP.
For instance, if you have additional app functionality that is now only available with the Windows Phone 8 platform, you can highlight those in the Windows Phone 8 app long description and maintain the applicable metadata long description for the Windows Phone 7 XAP.
If you currently have a Windows Phone 7.x app that you’re thinking about updating to take advantage of Windows Phone 8 functionality, you have some decisions to make about support for your existing Windows Phone 7.x apps.To better serve your needs, the Windows Phone Dev Center now offers the ability to associate multiple XAPs with the same app GUID.And you don’t have to worry about your app showing up twice or end users finding the wrong version for their device.Our catalog detection will show your 7.x app to those with Windows Phone 7 devices and the 8.0 app to those with Windows Phone 8 devices.
Here’s how to decide on your approach: If you want to have only a version of your app for Windows Phone 8 devices, you have to upgrade your existing Windows Phone 7 app to Windows Phone 8, or create a new one for Windows Phone 8.
This app will not be shown to Windows Phone 7 users.