Unable to Restore non-renewable Purchases

Hi all, I need help in Restoring subscriptions on iOS.

Lets consider a few cases:

Case1:
User Installs an application.
User purchases a non-renewable subscription for 6 months.
User uninstalls the application.
User Re-Installs the application

So how can I track that?
RestorePurchases restores nothing since they are non-renewable and Apple says that If your app uses non-renewing subscriptions, your app is responsible for the restoration process.

Case2:
Let's ASSUME I already have an application on App Store which has non-renewable subscriptions and many users have already subscribed for the "non-renewing" 6 months subscription.
And later I update my application and on App Store.
So after the Users update the application on their devices, will the subscription remain active?
If No, is there a way to restore their purchases? What else can be done?

Thanks in advance.