Microsoft says the year-long offer will expire on July 29, 2016, which is less than three months from today. After that date, unless a new offer of some sort materializes, you'll need to pay for a Windows 10 upgrade at the prevailing retail price, $119 for a Home upgrade or $199 for the Pro edition.
But what if you're not ready to switch to Windows 10 yet? Can you claim your free upgrade and keep using your current Windows version?
Yes, you can. And it's not a loophole, nor does it involve any tricks. It merely involves claiming your entitlement under the free upgrade offer and then rolling back to your prior operating system. With that entitlement in place, you can schedule the final upgrade for when you're ready, even if that's after July 29.
As with anything that involves Windows licensing, Microsoft doesn't go out of its way to explain these nuances of the Windows 10 upgrade offer. I am not a lawyer, and this post should not be considered legal advice. With that disclaimer out of the way, here's what you need to know.
First, this offer applies to anyone who owns a "qualified and genuine" device running a non-Enterprise edition of Windows 7 or Windows 8.1. That includes PCs you purchased with either operating system preinstalled as well as those that you upgraded using a copy of Windows you purchased from a reseller or from Microsoft directly.
Second, the upgrade process uses that underlying license to record a "digital entitlement" on Microsoft's activation servers: "Once you upgrade, you have Windows 10 for free on that device." The point of the digital entitlement is to associate the Windows 10 license with your specific hardware, making a product key unnecessary.
If you look in Settings > Update & security > Activation, you'll see this on a system that's been upgraded this way:
That dialog box is your confirmation that you have successfully recorded your Windows 10 digital entitlement for that device. Here's what I wrote last August about this these new activation rules:
When you upgrade from Windows 7 or Windows 8.1, the Windows 10 setup program checks your current activation status and reports the result to the activation servers. If you're "genuine" (that is, properly activated), the Windows activation server generates a Windows 10 license certificate (Microsoft calls it a "digital entitlement") and stores it in conjunction with your installation ID and the version you just activated (Home or Pro).
It didn't need a product key to do that activation. All it needed was the proof from the Software Licensing Manager utility that your underlying activation was legit.
You can now wipe that hard disk completely, boot from Windows 10 installation media, and install a squeaky clean copy.
You can also temporarily undo the upgrade and restore the underlying Windows 7 or Windows 8.1 license that you used to qualify the upgrade. That won't affect the digital entitlement you received, which remains safely locked up on Microsoft's activation servers, ready for you to use again when you're ready. How and when you choose to reinstall Windows 10 on that hardware doesn't matter: You can use a USB flash drive or a DVD, a mounted ISO file, or even run Setup from a shared folder on your local network.
If you use the Get Windows 10 tool to install the upgrade, you only have to run Windows 10 long enough to confirm that your license is recorded. As soon as that's done, go to Settings > Update & security > Recovery, where you'll find this option:
Click Go back to Windows 7 (or Windows 8.1, if that's what you're upgrading from) and the rollback process starts immediately. Because of the way the modern Windows setup program works, it's quick and extremely reliable.
The entire previous version is stored in its own folder, untouched. When I tested the rollback process, it took a mere seven minutes to wipe out the Windows 10 upgrade and restore the previous Windows version, completely intact.
If you're a belt-and-suspenders type, you probably want to do a complete image backup before you try this procedure. That way you're covered if anything goes wrong.
But the bottom line is this: If you think you might want to upgrade a PC to Windows 10 at some time in the future, claim that free upgrade now, before July 29 rolls around and the price goes up.