This is great, something Microsoft should have done a long time ago IMO. At last people can simply download an ISO of Windows to reinstall their machine rather than having to worry about making restore disks. It also resolves the issue where for Windows 10 you previously needed to use their pointless "Media creation tool" - All this did was mean that you needed a Windows machine to make an installer for Windows (can you see the problem with this!
).
As far as what happens if you have performed an upgrade from 7/8 - Your key should still work, I've clean installed Windows 10 on several PCs with Windows 7 licences and the 10 installer simply accepts the Windows 7 key.
If you don't enter a product key it will run
for a short period of time. In order to keep using it you will need to enter a product key.