How to test paid user is premium before releasing when moving paid app to subscription

I'm moving from my paid iOS app to a subscription model. I synced the build number of the paid app using the purchaseHelper.  https://github.com/codykerns/PurchasesHelper

CompatibilityAccessManager.shared.syncReceiptIfNeededAndRegister(entitlements: [

            .init(entitlement: "premium", compatibleVersions: ["1", "2", "3","4","5"])

        ])

How can I test that existing paying users are converted to Premium before releasing the app?

If I buy a paid app on my device and update it to a test flight, will I be able to test it?