As of March 1st 2014, The Windows 8.1 Enterprise Upgrade is now available as a standalone SKU in the Open and Select Volume Licensing programs.
What is changing:
• Windows Pro Upgrade: The Windows Pro Upgrade remains available as a License only, without Software Assurance (SA).
• Windows Enterprise Upgrade: The Windows Enterprise Upgrade is offered as of March 1, 2014 and allows customers with a qualifying OS to upgrade to Windows Enterprise with or without purchasing SA.
• Software Assurance: Except as stated below, SA may only be purchased for a Windows Enterprise Upgrade license.
Select Plus, Open Value, and Open License Programs: Customers may continue to acquire SA for new devices purchased through June 30, 2014 with OEM or FPP licenses for Windows 8.1 or 8 Pro or Windows 7 Professional within 90 days of purchase.
For example: If you purchase a new device with Windows 8.1 Pro on June 6th, 2014, you can attach Windows SA to the device without buying the underlying Enterprise upgrade license until September 4th, 2014.
Renewals: Customers who bought SA for Windows Pro may renew SA on their covered devices without buying a Windows Enterprise Upgrade license.
Purchasing Windows Upgrade + SA: Any customer in a volume licensing program that requires SA and who previously purchased Windows Pro Upgrade + SA may continue to purchase Pro Upgrade + SA until the end of their enrollment or agreement. Upon entering a new enrollment or agreement the customer will purchase Enterprise Upgrade + SA.