Merging events retrospectively

Hi community! Happy to be here!

I have a setup where I pull together events from website and events from an application (two separate sources).

Everything seems to work, the only thing I’m struggling with is that I don’t know how to merge the website events (by the device_id) retrospectively into a new user_id.

Example:

1. A new user signs up and does something in the app → a user_id #1 is created (= new account in the app) and the events from the website (“Try free” click, “Sign-up form”, etc) are attributed to user_id #1
2. The user logs out and leaves
3. The same user returns in the same browser and creates a new account (goes through p.1 with new credentials) → a user_id #2 with the device_id #1 (same as in p.1)

What ends up happening is the website events in p.3 (“Try free” click, “Sign-up form”, etc) are attributed to user_id #1 even after user_id #2 is created.

Question: how do I make the website events after the logout merged into the user_id #2 by the device_id #1?

I’d appreciate any help!