You are likely to run into the following error during the upgrade process:
Windows 10 installation failed in SAFE_OS phas with error during boot
The error code reported to you when you reboot the system and it rolls back changes into the old OS is likely:
0xC1900101 – 0x20017
This appears to occur because Windows doesn’t care for the QEMU-provided CPU unless you’re using (as far as I am aware) the core2duo emulation option.
So, simply shut down the guest domain, choose “core2duo” as the CPU type, downgrade your processor count to 2 (if necessary), and retry the operation. It succeeded for me! After the upgrade, it seems you can reset the CPU to host (or whatever you use) and increase the processor count without issue.
Additional note: If you’re using the Red hat VirtIO drivers in Windows 7, Windows 10 will work with those drivers during the upgrade process without an issue.