How to keep App User ID

Hi guys, 

I'm trying to understand how to deal with "App User ID" and user accounts.

All good if I have the next flow:

1. A user downloads an app.
2. The user register in the app
3. Based on the registered user account, I can generate an "App User ID" and send it to RevenueCat.
4. Then, based on the registered user account, I always have access to the user "App User ID" because it is stored in my database and mapped to the registered user account.
5. Later on, users can restore purchases anytime after they log in to the app. Because again, I have his "App User ID" stored in my database.

But what should I do if I have no user registration in the first place? Is it possible? 
 

For example:

1. A user downloads an app.
2. I generate some random "App User ID" based on the device UUIDD
3. The user goes through onboarding screens with a PayWall in the end.
4. The user makes a purchase. 
5. All that I have now, it's his "App User ID" mapped to his device UUID, and that is it. If the user changes the phone or tries to log in on other platforms, I wouldn't be able t identify him because he has no user account in my database. 

So, I must somehow register the user at first to be able to map the "App User ID" with some account in my system, right? Or is there another way? Is it okay to create a user account after purchase?

​​​​​​​Thank You for your support!