I do own a small refurb firm, dealing mostly with Dell/Fujitsu/HP/Lenovo business-line laptops and desktops.
These machines were (almost exclusively) shipped with a 7/8/8.1 Pro license.
Though the official upgrade path has been closed long time ago, I'm still acquiring Windows 10 Pro digital licenses.
I'm using sysprepped (and generalized) images via Windows Deployment Services.
If the target machine bears a Windows 7 Professional COA, all you have to do is "Change Product Key" in the Activation window, type in, and activate, even after a clean install (image) on hardware that never acquired a Windows 10 digital license before. Dell, Lenovo and some Fujitsu do activate successfully even if you're using another (bit-clone) imaging solution like Acronis...HP machines don't like cloning when it comes to activation - use WDS instead.
If the machine bears a Windows 8 Pro sticker - there's no product key you could enter, and the OEM keys retrieved with known tools don't work either in most cases. There are numerous possibilities, depending on whether the machine has been shipped with (or has been made ready for) Windows 8.1.:
1. The machine has been (UEFI/MSDM) licensed for Windows 8 (not Windows 8.1):
These are the rigs that won't ask for a product key if you dare to install Windows 8, but they will ask for one if you want to install Windows 8.1 (and Windows 10). You
have to install Windows 8, upgrade from Windows 8 to Windows 8.1 then upgrade to Windows 10. This gets a digital license for the hardware, and you can clean install (image) any Windows 10 build on it after that.
BUT... I have encountered some mood swings in case of Windows 10 versions (builds); recently, I've had some Dell OptiPlex 7010 Desktops (3rd Gen, with recent UEFI/BIOS, SLIC2.1) which behaved strangely:
- Windows 8.1 doesn't ask for a product key
- But any Windows 10 version does ask for a product key
- Upgrade from Windows 7(SLIC)/8.1 to Windows 10 up to v1709 works flawlessly and acquires a digital license for the machine
- Upgrade from Windows 7(SLIC)/8.1 to Windows 10 v1803 no longer works - breaks with a "your product key couldn't be verified" message. Naturally, the path is to upgrade to v1709, acquire digital license, then install (image) v1803 - and it will be activated.
Alternative: Such machines mostly do have a SLIC2.1 module embedded in BIOS, so you can install a Windows 7 OEM image, then upgrade right away to Windows 10.This gets a digital license for the hardware, and you can clean install (image) any Windows 10 build on it after that.
2. The machine has been (UEFI/MSDM) licensed for Windows 8.1
In MOST cases, Windows 10 (and indeed, Windows 8.1) will clean install (image) and activate right away, without asking for a key.
I'm not considering native OEM Windows 10 rigs - if the machine bears a Windows Pro sticker (without the "8") - you're almost sure your machine already has a license for Windows 10.