If for some reason you need to change your Apple ID you can download Youper using a new/other Apple ID. After the updating you would need to log in to Youper using your same email address that you originally used to create your Youper account.
If for some reason, you don't have access to the premium content, you can restore the subscription and everything should work fine.
Click here to access an article that explains on how to restore your subscription.
If you are having difficulties doing so, please reach out to our support team.