Windows 8.1 fresh install with Windows 8 licence

Here are the steps that lets you download the Windows 8.1 ISO using a legitimate retail Windows 8 product key:

• Download both the Windows 8 and Windows 8.1 setup installers from here.
• Once downloaded launch the Windows 8 installer and enter your product key when prompted to do so.
• Once the download begins, immediately pause your download at 1%. Do not quit the installer at the estimating time screen. Once you pause the download at 1%, quit the installer.
• Now, run the Windows 8.1 installer and you will notice that it will tell you "download did not complete successfully" and that there was an "element not found" error. This is normal. Close the installer.
• Now, re-launch the same Windows 8.1 installer. You will now be downloading the Windows 8.1 ISO. After the download is complete, you can save it as an ISO or create the USB media immediately.
• You can install Windows 8.1 using the ISO or even burn it to a DVD or make a bootable USB thumb drive with the media.

There is also an accompanying youtube video to the aforementioned step by step instructions that can be found alongside the full article entitled "How to download the Windows 8.1 ISO using your Windows 8 retail key" here.

The Windows 8.1 installer from the ISO does not accept Windows 8 keys. You need to install the system with a generic key (334NH-RXG76-64THK-C7CKG-D3VPT for Core, XHQ8N-C3MCJ-RQXB6-WCHYG-C9WKB for Professional) and then enter your Windows 8 key after installing.

_{Note to moderators: these are not usable Windows keys. They are embedded in Windows itself as a placeholder default key and do not let you activate.}

If you don't have access to an MSDN or Technet subscription or you're not a volume licence customer, the only way to get your hands on an ISO is to download it from somewhere else. Microsoft's own server is IMHO far better than any torrent site. Contrary to what Microsoft wants you to believe, you can use the generic key provided in this thread, download the ISO and burn it to a blank DVD from their own site. Boot your system with the DVD you just burned, enter the same generic key to install and proceed on. Activate your Windows installation afterwards with your own Windows 8 key.

Then why not just use the Store? Using the official method you end up with no install media, no product key and no way to use the built-in recovery later on ('missing files' error). Microsoft really messed this up by not providing an official way to do a fresh installation from an ISO for free upgrades.

Microsoft installer and ISO download

Addendum:

The generic key, although accepted as 'working' in the installer, won't start downloading the ISO -- an error message citing "We cannot connect now. Please try again later." is displayed forever no matter how many times you try. I found a solution to this. Download both the Windows 8 installer (Windows8-Setup.exe) and the Windows 8.1 installer (WindowsSetupBox.exe) from Microsoft's site. Run the Win 8 installer and start downloading but cancel it by closing the program. Now, start the Win 8.1 installer and it will accept your Windows 8 key and start downloading the new 8.1 ISO.

If you're an existing Windows 8 user, updating to 8.1 should be relatively easy, via Windows update. Microsoft also says that existing users with Windows 8 licence cannot clean install 8.1 using that key and will need to reinstall their Windows 8 before moving to 8.1. Otherway, they have to purchase a new key.

I'm running the Win 8 Enterprise, and we were able to activate clean Win 8.1 installations with Win 8 key. So I suppose that new key requirement is valid only for OEM and Retail Windows 8.1

You can actually download the single language ISOs directly.

Just head over to https://www.microsoft.com/en-us/software-download/windows8ISO (from Mac or Linux, or change your user agent so it doesn't redirect you to download manager), and download the ISO. I've just updated my W8 oem laptop's license to W10 that way. Just don't add any CD key whenever asked, it both acticates W8.1, and (after official upgrade procedure) W10.