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From WiX Wiki at MindCapers

Awesome explanation.

I only had to tweak it to get it to work. I used WiX 2.0.5805.0, and changed the registry entry into:

<Registry Root="HKCU" Key="SOFTWARE\Blah\MoreBlah\BlahdeeBlah" KeyPath="yes" Type="string" Value="1.0.0" />

Thanks!