Thanks for posting this little nugget. It looks to be a great find.
I created a flash drive with the 8.1 setup on there as per the instructions in the link above. The trick is to create that ei.cfg file and make sure it matches your version of Windows 8. You put this file into the sources folder on the flash drive. The 8.1 iso download has all versions, so this ei.cfg needs to be edited to tell it what version your Windows 8 product key is for.
I installed a clean Windows 8 on a computer from a DVD and entered the Windows 8 product key. It had no updates at all and then I put the flash drive in and ran the 8.1 install setup from within the Windows 8 environment to start the upgrade. It did not prompt me for a product key during this installation part of the upgrade. The installer asked me if I wanted to keep my programs and files. It then proceeded to upgrade my Windows 8 to 8.1.
The first thing that came up after the upgrade during the OOBE (out of box experience) was the Product Key page. You can now enter the Windows 8 product key or choose to SKIP it and enter it later. Windows 8 did not have the option to SKIP the product key. I chose to SKIP it.
A short while later, the Windows 8.1 GUI was displayed. I entered the Activate Windows area and entered my Windows 8 product key in and it activated it perfectly.
There are four benefits to this method above.
1. When upgrading computers from Windows 8 to 8.1, you no longer have to make sure each machine has all the Windows 8 updates installed. Huge time saver.
2. The second benefit is that you don't have to download Windows 8.1 upgrade which is 3+ GB from the Microsoft Store on EACH computer you need to upgrade to 8.1. Huge time saver again and saves bandwidth.
3. No need for a Windows 8.1 specific key to make this upgrade happen. Nor should there be since Windows 8.1 is a free upgrade.
4. The ability to SKIP the product key entry during the OOBE means you can get Windows 8.1 installed and ready to go and then add the product key later. Windows 7 had this ability but 8 hasn't until now.
I will try installing a clean Windows 8.1 from the flash drive next and see if it will let me with the Windows 8 product key and report back. It would appear that the Windows 8 > 8.1 upgrade has been made much easier for techs with this trick.
