http://www.cnet.com/how-to/5-reasons-to-upgrade-to-windows-10-right-now/

You can change your mind

If you upgrade to Windows 10 and decide that the new operating system isn't for you, you have 30 days to roll back to your older version of Windows. And by upgrading once -- even if you downgrade later -- you'll have secured the free Windows 10 license for future upgrades. In other words, you can upgrade to Windows 10, roll back to your old operating system, and then you'll still be able to upgrade to Windows for free after July 29.

Good to know.