I was wondering what edition of Windows 10 i'll probably get for free, turns out as a current Windows 8.1 Pro user, i'll get Windows 10 Pro. It's a like for like upgrade, so for example if you are using Windows 7 up to Home Premium, you'll get the Home Edition. Anything higher, Pro.
I remember getting Windows 7 Home Premium for £50 on my way home from work years ago. Couldn't believe the price, thought it was a mistake. But it was right. And a few years later, got a Windows 8 Pro Product Key for £25 direct from Microsoft as part of their reduced price upgrade offer. So Windows 10 Pro for free, can't fault it.