The Old with the New - How to [unofficially] upgrade from Windows 8.1 Preview to Windows 8.1 RTM [Think "cversion.ini hacking for fun and..."]

Do This Best - How to Site - How to Upgrade from Windows 8.1 Preview to RTM – Guide

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One of the problems users encountered with these leaked ISO is the fact that they require a clean install, so everybody would lose all settings and apps.

The good news is that upgrading from Windows 8.1 Preview to RTM is actually possible, even though you install the stable version of the operating system from a stand-alone ISO.

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1. Download Windows 8.1 RTM ISO

2. Modify cversion.ini file

3. Save Windows 8.1 RTM ISO

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In the end, you only need to burn the ISO to a DVD or create a USB stick to start the installation of your Windows 8.1 RTM OS. After launch, the installer will give you the option to either deploy a fresh copy of the operating system or upgrade from Windows 8.1 Preview, while keeping settings and applications

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It's an somewhat old hack, but has been know to work. For the record, I would NOT suggest you get a Windows 8.1 RTM ISO from another other source than Microsoft (i.e. just hold your horses until 10/18). You grab an leaked ISO, you're taking a huge risk. Don't do it. It's only a month and a half...

But when you do, this hack might come in handy. Remember YMMV, and this is totally unofficial and not supported (and don't come crying to me if this hoses your PC, etc, etc, etc).