I don't know what method Microsoft will use to deploy free Windows 10 to participating Windows machines.
I just upgraded a Windows 8 machine to Windows 8.1 - by way of the green Windows Store tile. It took a while to download 3+ Gb of Windows 8.1, then took its time to install in-place over Windows 8.
After it all settled down after rebooting, we have a Windows.old folder on C: drive. I'm pretty confident I don't need it anymore but it won't delete using Windows Explorer - it gets to the Windows.old\System folder and stalls because of some access rights / permissions. It does not ask for an admin password so I assume that it is some system folder permissions issue with a dodgy no can do error message. I thought, "ugh! do I have to go permissions hacking?"
Turns out, that's not how to delete Windows.old According to Microsoft article How Do I remove the Windows.old folder, I can use the Disk Cleanup tool.
And then I looked at the Disk Cleanup tool and instead of ignoring the short list of boring things it clears (which often does not do enough when I am desperate), there is actually a list of interesting things it can clear off.