How to activate Windows 10 after a reinstall on (Tier 1) OEM licensed hardware

I've recently been buying a lot of Dell hardware for my clients, then performing aftermarket upgrades as it is very cost effective. After installing a new SSD and reinstalling Windows, it reports as not activated.

To fix this, run the following command:

wmic path SoftwareLicensingService get OA3xOriginalProductKey

It outputs the key straight away, and you can copy/paste this to the regular activation window.

And done - this is ridiculously simple and I am not at all sure why Windows doesn't do this automatically.

(I usually install from a ISO acquired from VLSC. I have not tested using a Dell branded disk, or even a standard OEM edition)

=====update=====

I had a client purchase 10 machines that were upgraded - 9 worked fine, then the 10th said that the key had already been used (I did the above steps each time, didn't reuse once).

The solution was to go to the (elevated) command prompt and type "slui 4"

I then did the old phone activation which went through fine.

I have no idea why 9 were successful and 1 failed.