I have app(silverlight) for Windows Phone 8 and higher. I will update this app to Windows Phone 8.1 (RT). In WinRt in app purchase changed. How do I migrate purchases of user from the old version app(silverligth) to new version app(WinRt)?
You can use Windows Store collection API to get all the products owned by a given user.
You can also follow Query for products to scope your query to your particular product. After getting the list of user who has purchased the product before, you need to make configuration to make your IAP feature still available to them while other users still need to click "purchase" button to unlock those features.
For more details, see View and grant products from a service.