I did it but you have to trick the downloader to download 8.1 by using a windows 8 key. Then if want to able to use the subsequent burned disc to install 8.1 you have to edit the iso by adding a small text file that also lets you install 8.1 and activate it with your valid Win8 key. Until or even if MS decides to let holders of valid Win8 licenses download the 8.1 iso file this is the only way to get it. I'll post a link to the site that shows you how to do it.
http://www.neowin.net/news/here-is-how-to-get-the-windows-81-iso-and-create-a-usb-install-stickIt will work to upgrade from 8 to 8.1 only and can be useful if you have more than one machine that needs to be upgraded since you won't have to do the download and install from the Windows Store again.
Now if you want to be able to use the usb stick or DVD to do a fresh install of 8.1 you need to do this.
Extract the iso file with something like 7-Zip and add a small text file called ei.cfg to the sources folder. This is what this will do.
you can create a new file in the sources folder called ei.cfg with the following in it:
[EditionID]
Professional
[Channel]
Retail
[VL]
0
This will make it ask for a key after installation instead of before, and it accepts windows 8 keys.
The file can be created in Notepad. You then will have to burn the iso again to a new disk or you can do the edit before burning it to disk or USB stick. It also only works if you have the pro version. If you have the standard core Win8 then type in Core instead of Professional. This is all legal by the way since MS is the source of the .cfg file and you still need a valid Win8 key to make it work.
