There are two separate downloads for customers with a later license who want to obtain XE8 as a previous version, depending if they have update subscription or not. We recommend you download the appropriate link for a successful installation:
XE8 General Update can be downloaded if you do not have a current Update Subscription
XE8 Subscription Update can be downloaded if you do have a current Update Subscription
Serial Number for XE8 Installation
Installation of XE8 must occur on the same machine as your 10 Seattle installation. Use your 10 Seattle serial number to generate a serial number used for installation of XE8 here
This is a throw away serial number for installation only, and it is important you use one matching the product version you are licensed to, or the later registration will fail.