I have an Windows Phone app designed to work on WP 7.5. It was uploaded to the store long time ago and now i wanna update the app. I want to update the app for WP 7.5 and also create the same app for WP8. I'm doing the updates to the existing WP7.5 project and get the XAP file and then Upgrade the project to WP8 and the do some changes and get another XAP for windows phone 8. But my question is how can i upload both XAP file to the marketplace. Im not very clear about what should i do. if i do the above procedure, would i be wasting my time. I have another app that i updated for WP8 but not for WP7.5 my nokia Lumia 710 shows the older WP7.5 version of the app. I understand the logic behind that but can i update for both of the OS at the same time?
I'm not sure what you mean. Basically, you have to upload a separate XAP for WP7 and for WP8. I recommend keeping a copy of your WP7 Visual Studio project before upgrading it, just in case you want to publish a bug fix or something for the WP7 version of your app.
In the DevCenter, when submitting an update, all the XAPs available for your application are listed. You can click on "Add new" to add a XAP for a different version of the OS. Use "Replace" to update an old XAP for a specific version of the OS. This way, you can independently update your WP7 and WP8 versions. You can't update both at once (meaning, upload a single XAP and use it for both versions of the OS) since WP7 isn't forward-compatible