Microsoft already said that it would upgrade Windows 7 and Windows 8.1 customers for free to Windows 10, but now the company is clarifying the upgrade paths, in a document released Thursday at its WinHec hardware partner conference currently taking place in Shenzhen, China. Chances are your machine is eligible—if you’ve kept it up to date. But certain devices will be left out.

If you own a computer running the most recent update to Windows 7 or Windows 8.1, you'll be able to download Windows 10 when the highly anticipated...