Hey there,
When the same wearable account is connected again under the same dev_id, Terra handles everything automatically: the previous User record is deleted, and a brand-new User with a new user_id is created. This ensures there is always only one active Terra User per wearable account for your dev_id.
The user_reauth event is different. It fires when the same Terra User reconnects their wearable, not when a wearable account gets connected to a different end user in your system.
To track this cleanly on your side, use the reference_id you pass during authentication. When you receive a new successful auth event, check if that wearable account was previously tied to a different reference_id. Terra will already have removed the old User, so all future data will automatically flow to the new user_id.