It's also known as being not true, as on desktop people are attaching this vdisk to virtual machine, running upgrade, and after booting real machine from it... well, it works. But there's no ARM VM to do that - still I think there's other way to do it.
Seems that very recent version of dism tool from leaked 8.1 2014 upgrade supports ESD files. So what I'm thinking about is upgrading image directly using it, on desktop.
But I'm looking for a way to obtain ESD file. When I started upgrade from Windows Store, in \Windows\Software Distribution\Downloads I found only .exe file responsible for upgrade, but not .esd itself. Any hints where it's located? Or anyone can share it?