Microsoft's free upgrade offer to Windows 10 stands until July 2016. Customers who work on genuine Windows 7 or Windows 8.1 systems may upgrade those systems to Microsoft's newest operating system Windows 10 free of charge.
It does not matter whether the source operating system is an OEM or Retail version, as both upgrade just fine to the new OS.
Retail versions offer the advantage that they allow you to change hardware of the PC without losing your license.
The operating system will either activate automatically after the hardware swap or after contacting Microsoft.
OEM versions on the other hand allow only some hardware changes, say an increase in RAM, but not major replacements like a new motherboard.
A report on the German tech news site
Ht4u suggests that the free upgrade to Windows 10 will turn retail licenses into OEM licenses that are linked to the machine.
I tried to verify this by checking the status of a retail system that I upgraded to Windows 10, but at least for the moment, it shows as a retail copy.
