You need to unlink the device from your new account and activate it with your old account.
1) Go to https://my.roku.com/account with the new account email and click on "unlink" for the device.
2) Log out of https://my.roku.com/account with the new account, and restart the device.
3) When activating your device again and providing an email, provide the email of the original/old Roku account, and login to https://my.roku.com/account with the old email address.