Windows 10 is here, for better or worse. As the family sysadmin, I've been tasked to update the Windows machines: ultimately, failure modes are not well documented and I needed Free software and several hours to recover a vital machine.
The "free upgrade for users of prior Windows versions" is a limited time offer for a year from launch. Microsoft do not offer licence keys for the upgrade: once a machine has updated to Windows 10 and authenticated on the 'Net, then a machine can be re-installed and will be regarded by Microsoft as pre-authorised. Users don't get the key at any point.
Although Microsoft have pushed the fact that this can be done through Windows Update, there is the option to use Microsoft's Media Creation tool to do the upgrade directly on the Windows machine concerned. This would be necessary to get the machine to upgrade and register before a full clean install of Windows 10 from media.
This Media Creation Tool failed for me on three machines with "Unable to access System reserved partition'
All the machines have SSDs from various makers: a colleague suggested that resizing the partition might enable the upgrade to continue. Of the machines that failed, all were running Windows 7 - two were running using BIOS, one using UEFI boot on a machine with no Legacy/CSM mode.
Using GParted live .iso - itself based on Debian Live - allowed me to resize the System partition from 100MiB to 200MiB by moving the Windows partition but Windows became unbootable.
In two cases, I was able to boot from DVD Windows installation media and make Windows bootable again at which point the Microsoft Media Creation Tool could be used to install Windows 10
The UEFI boot machine proved more problematic: I had to create a Windows 7 System Repair disk and repair Windows booting before Windows 10 could proceed.
My Windows-using colleaague had used only Windows-based recovery disks: using Linux tools allowed me to repair Windows installations I couldn't boot