Word of warning!
I tried four times to ugprade my PC from Windows 8.1 Pro to Windows 10 Pro.
I had all Windows updates installed, everything was up to date and perfect going into the upgrade.
I had reserved my upgrade last week, but decided I couldn't wait.
So I did each of these three methods, with the exact same result:
1) Media Creation Tool -> Upgrade to Windows 10.
Failed at 60% during the "installing devices and features" phase. Error upon reboot (and rollback to windows 8.1) was "Error during FIRST_BOOT during DATA_MIGRATION phase".
2) Registry edit to allow for update to occur via windows update window.
Failed at 62%, same spot, same error on rollback.
3) Downloaded Windows 10 x64 ISO file, mounted, upgrade to windows 10.
Failed at 59%, same spot, same error on rollback.
Before trying the third option (ISO), I uninstalled my NVIDIA drivers and reinstalled the latest, as well as disconnected EVERY USB device save for my mouse and keyboard, all secondary SATA hard drives, and it still failed.
I ended up doing a clean install, as much as I didn't really want to.. but it worked.
but if you're having the same issues via your upgrade, I wasn't able to figure out how to successfully upgrade my machine without a full install.
I am on a 128GB SSD, I don't know if that contributed to the issues or not, but I don't see why it would.