Microsoft will allow users of PCs and tablets running Windows 7 and Windows 8.1 - along with users of Windows Phone running 8.1 - to upgrade to the new Windows 10 OS free of charge for the first year after Windows 10 launches. It's a radical change from the way Microsoft's operated in the past; none of this Windows Home, Windows Professional, and varying upgrade costs - it's just going to be one free upgrade per device.
The idea seems to be to encourage widespread and rapid adoption of their new OS while reducing support costs associated with the various flavors of the current Windows OS milleu. Of course, folks running Windows 7 on older systems may encounter some hardware issues, although if they've updated some of their hardware over time, they should find migration relatively painless. For users of Windows 8.1, the upgrade's expected to be simple and straightforward.
If anyone's still dinking around with Windows 8.0, I suggest upgrading to version 8.1, which is also pretty easy. And you'll like the interface as well.