Here is my experience.

Made 10166 image of C:\ with Macrium Reflect.

Downloaded the 10240 upgrade file

Did not upgrade after files downloaded.

Created an iso from install.esd file

Instructions on how to do it :-

http://www.zdnet.com/article/how-to-...e-into-an-iso/

Created a bootable USB

Clean installed from USB.

Needed windows key.

Tried win 8.1 key. No go!

The installation was crippled. Could not use settings

Next step

Restored my 10166 image made with macrium

Upgraded it with 10240 bootable usb

Clean install again from from bootable usb

After installation windows was automatically activated

System information

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windows 10 Pro

Windows is activated
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HS