Migrate from the legacy 2 app model (free app + paid "unlocker" app) to in-app payment / billing library?

Question

I have 2 apps - users typically install free MAIN app, and if they want, they can purchase paid UNLOCKER app, which is nothing but a dummy apk serving as a license.

In my MAIN app I am simply doing this check:

if (packageManager.checkSignatures (
    mainPackage, unlockerPackage == PackageManager.SIGNATURE_MATCH {
// ... unlock app
}

It's the same business model as KWGT widget - this is main app and this is unlocker app

Now, is there an easy way how to migrate to IAP payment and get rid of dummy UNLOCKER app? Problem is I don't know how to distinguish between:

• "new" users who never purchased my app (these should be asked to pay with IAP)
• "old" users who had already purchased the app, but switched their devices (these should asked to install UNLOCKER again as I'm doing now, or grant them some "dummy" zero IAP purchase)

1. Is there a way how to programmatically check ur using some billing library call, if a user ever paid for my UNLOCKER app from within MAIN app?

PS: I have already 10 000+ legacy purchases in the past 5 years, so it's not feasible to send each and every one a PROMO code or support them by mail. It would be way too complicated + I am afraid of receiving bad ratings.