As part of windows 8 upgrade to 8.1, if you do a fresh install of Microsoft Windows 8.1 using Windows 8 key, one issue you may run into is you may not have your Windows 8 product key handy. If you still have your old system running, you might be able to extract your Windows 8 (or 8.1) key from the registry by using a key finder software, so you should do this step first before installing 8.1 over the existing Windows 8.
There are many key finder software available, but I find Belarc Advisor (not affiliated) to work very well for this purpose. The program will create a detailed profile of your PC, including installed software, and save it in an HTML file to your local hard drive. Once profiling is complete, the HTML will automatically open in your browser. Click on Software Licenses on the left of the page to see your software.
On Software Licenses section, you should see most of your installed software listed along with the product keys. Not all product keys will be displayed, fortunately, Windows 8 product key is among the listed.
Write down the Windows 8 product key and save it for your Windows 8.1 activation after installing it using KMS Key.
As you might have figured this out, this trick works not only for a windows upgrade, but you can also use this to reinstall windows 8.1 (or 8) without the serial number handy.
The FREE version (for personal use only) of Belarc Advisor is more than sufficient for this task and rest assured it does not send up your PC profile to a Web server across the Internet.
This solution does not always work, but I’ve seen good results in most cases. So I think it’s worth a try before you do anything more involved.