I have done it already. I had forgotten the F1 was on TV today, so I had some time to reinstall my PC while watching the F1.
I can tell you, the installation I explained works on a complete empty hard disk. I had removed my partitions from the hard disk and then when I tried to boot my PC, it gave an error message. The PC couldn't find anything to start with, of course.
I then booted from my USB stick with the Windows 8.1 Pro 64bit ISO installed on it and it started the installation.
With the ei.cfg installed in the source folder, I didn't need to input a Windows 8.1 key.
After the installation was done, it asked me for my key and I entered the Windows 8 Pro key from the upgrade disc I have bought and then it went on with preparing my system for its first start.
After it had started, I noticed my Windows 8.1 Pro wasn't activated, but I just needed to call a local free number and go through the standard procedure to confirm that this key wasn't being used on another PC.
And my freshly clean Windows installation is now ready on my PC.
(The other software I use I'm going to install tomorrow.)
There is also something else I want to add. Now that I did this install, I paid a little more attention to the process.
At a certain point, you can choose for an upgrade or custom installation. I needed to choose the custom, of course. But for fun, I also tried the upgrade. I then got a message that I needed to start the PC with my installed OS, and from there, run the setup of Windows 8.1 Pro.
So, for an upgrade, you don't need to boot from the USB stick, but you need to boot your current OS and start the setup from there.
Because I had never done an upgrade, I didn't knew this, but I just wanted to give you this information.
I hope this will help you prepare your system!
Best regards,
Rudy