Is it? If you use the Microsoft provided media creation tool for Windows 10, it will automatically pick up OEM BIOS keys (including from earlier versions of Windows) and install the appropriately licensed version.
I did it last week on a laptop that I put in a new SSD in and hasn't had a Windows install in over a year (but came with Windows 8 installed when I bought it). I really don't think Microsoft would go out of their way to make this possible if it was against their terms.
I did it last week on a laptop that I put in a new SSD in and hasn't had a Windows install in over a year (but came with Windows 8 installed when I bought it). I really don't think Microsoft would go out of their way to make this possible if it was against their terms.