While upgrading to Windows 8.1 is proving trouble-free for most users, Microsoft seems to have gone out of its way to make a few upgrade types difficult or impossible. For example, it's not possible to install Windows 8.1 using a Windows 8 product key, which makes no sense at all. Worse, Microsoft is punishing its most loyal users by refusing to provide a downloadable install set in the form of an ISO or similar archive, which would enable users to install or upgrade to the OS multiple times with just a single download. Why make life so difficult, Microsoft?
Neither of these issues makes any sense at all.
If you bought Windows 8 in any way—as an upgrade from a previous version using retail or electronic media, as a full install electronically or retail, or with a new PC, you can get Windows 8.1 for free. So why wouldn't every single Windows 8 product key out there work with every single Windows 8.1 installer?
Consider the following: You as an individual may want to upgrade a single Windows 8 PC to Windows 8.1. So using the normal electronic upgrade process isn't particularly onerous, as outlined in my post Windows 8.1 Upgrade: Step-By-Step: Yes, you have to make a 2 GB to 3.6 GB download, depending on which version of Windows 8 (or RT) you're currently using, but you only have to do it once, and depending on your connection speed, the whole thing should take under an hour.
But what about the guy with 2, 3, 5 or more PCs to upgrade? In places where downloading is slow or expensive, asking such a person to redundantly download exactly the same code over and over again is a bit much. But that's exactly what Microsoft does.
What's interesting about this is that Microsoft offers a Windows 8.1 Upgrade Assistant which, like its predecessors, lets you create DVD- or USB-based install media. You just need to enter a valid product key to use it. A valid Windows 8.1 (not Windows 8) product key.
Likewise, a separate online tool, aimed specifically at those with product keys, is designed specifically for those who wish to download ISOs. But you must have a valid Windows 8.1 key to download a Windows 8.1 ISO.
Come on, guys. Seriously?
On a related note, Microsoft does make Windows 8.1 ISOs available via TechNet, MSDN, Volume Licensing, and perhaps other outlets. I've only tested the TechNet version—I pay for that subscription—and while you can in fact activate a TechNet-based Windows 8.1 install with a retail Windows 8 product key, you cannot in fact trigger Setup with such a key; you must instead use your TechNet key. (You can install Windows 8.1 offline and then change the key after the install is complete.) This suggest to me that making retail ISOs available to the public would be very simple. What I can't explain is why they won't do that.
Update: Thanks to feedback from many readers, I can report that there is a workaround that will let those with a valid Windows 8 product key download a Windows 8.1 ISO. You can read all about it in Windows 8.1 Tip: Download a Windows 8.1 ISO with a Windows 8 Product Key.