If you have Windows 7, 8 or 8.1 — even the Pro versions, as long as you bought them some other way than through a volume
licence — you'll get a
free upgrade to Windows 10 in the first year.
The
free Windows 10
upgrade doesn't apply to Windows Enterprise; that's because you can only get it with a volume
licence (and you have to already have a Windows Pro
licence for each PC), and if you have a volume
licence you already have the option of Software Assurance, which includes
upgrades.