Windows – Reinstalling a fresh Windows 7 on a SSD while its original HDD version was upgraded to Windows 10

hard drivereinstallssdwindows 10windows 7

Many posts around that subject, but this case is a bit specific (I think) since it's not about a migration nor a cloning.

• Initially Windows 7 on a 500GB HDD (OEM)
• then upgrade to Windows 10 (offered by Microsoft)

Now we have a brand new 550GB SSD and would like to install a fresh Windows on it (and get rid of the old one on the HDD).

• Do we have to install the initial (Win 7) or the upgraded (Win 10) on the SSD?
• What happens with the license, will Microsoft accept the license "transfer" automatically?

We'd prefer to install Windows 7, unless 10 is mandatory (or recommended).

Edit

Actually we'll go for win 10 since we need to switch languages after the install (thanks @Ramhound). Thus the question is back to the classical Win 10 reinstall, and is definitely a dupe . . .