I did the upgrade to Windows 8.1 Pro but not with a clean install. You can Upgrade Windows by using the "Add new features to windows 8.1" function in the control Panel. After putting in the Windows 8.1 Pro Key it comes a message that tells you if you want to upgrade to Windows 8.1 Pro. Some minutes later you have a Windows 8.1 Pro Stick. :-)
What Rico stated is correct, the only way to perform a healthy upgrade to Windows 8.1 Pro or Enterprise is by doing it through the default Windows updates feature. In the lower left corner of the Windows update page you find the "Add new features to windows 8.1" option.
Please bear in mind that Intel® Compute Sticks are compact computing solutions designed to work as they come from factory. They are not expected to be customized reason what the BIOS is not programmed to boot to any external media. Also be aware that the operating system version supported in these units is 32-bit so the upgrades will remain in the same 32-bit type.
To know more about supported operating system visit: http://www.intel.com/support/motherboards/desktop/sb/CS-035602.htm
And in case you decide to upgrade to Windows 10 use this other link for instructions:
I completely understand that, and that is what I was having issues with.
And technically what you said is NOT true. It WILL boot from external media, I did it several times. More consistently with a Linux based USB stick, but it booted from a CD Rom as well.
My issue was occurring when I was ADDING FEATURES function, and it gave me that error. The ever so informative "SOMETHING WENT WRONG" I was able to upgrade it though, by using that process, on my own, after finding the Operating System had an issue that was causing Windows to block the upgrade process, and stick, resulting in the something went wrong message.
The issue was fixed by killing several processes, and reformatting the OS and recovering the factory image, and before it connected to ANY updates, connecting it to the wifi and immediately running the upgrade process, which errored out, but when it rebooted, it was complete, and functioning.. and upgraded to Professional. It since has been added to the domain, and connected to servers to manage it, push out commands to it, powershell command tested, remote accessed, and ends up doing a LOT better than I expected.
I cannot upgrade it to Windows 10 as the domain Policies are for Windows 8.1, and that is our set standard currently. The Intel Compute stick at home, upgraded first thing out of the box to Windows 10, and functions well, but would do better with a NIC.
My biggest suggestion to Intel about future revisions are more than 1 USB port, and an embedded NIC. Even if the stick was slightly larger these would make a HUGE difference in the functionality of this device.