Upgrade Windows 8.1 SL (OEM) to Pro using MAK key

My organisation has quite a number of computers running Windows 8.1 SL (OEM) as it came installed when purchasing the machine. We have recently purchased a Volume License (MAK) Key in order to upgrade these computers to Window 8.1 Pro.
I click on "Get more features with a new edition of Windows" and enter the new MAK key and get the following message:  "This Key won't work. Check again or try a different key"

I have found a way to get our Windows 10 SL machines to work but that method does not work with Windows 8.1
We also have Windows 7 computers that I will need to do the same with.

How can I get this to work as I do not want to have to reinstall all these computers?

Thanks
Barry KaySystems EngineerAsked:
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McKnifeCommented:
Simply use an ISO with 8.1 pro to inplace upgrade (start setup from with indows - same bitness and language and edition required). You can keep your programs, files and settings AND the MAK key will be accepted.
James EdwardsCommented:
Windows MAK keys are volume license.  Volume license is Enterprise only.  You cannot cross from non-Enterprise to Enterprise on the upgrade paths.

8/8.1 upgrade path info can be found here: https://technet.microsoft.com/en-GB/library/jj203353.aspx

This also doesn't take in to account that upgrading from OEM to retail is also often not supported by MS, and then again adding edition changes back in to the equation.  Looks like a clean install for you methinks.

The most pertinent part of the link:

"Upgrade to Windows 8 Enterprise (Volume License)
You can upgrade to Windows 8 Enterprise (Volume License) from the following operating system editions:
Windows 7 Professional (Volume License)

Windows 7 Enterprise (Volume License)

Windows 8 (Volume License)"
McKnifeCommented:
No, MAK are not enterprise only, but also the pro edition can be aquired using a volume license (we do this) and with it comes a MAK key that you can use with any 8.1 pro setup ISO that I have encountered, so far.
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Barry KaySystems EngineerAuthor Commented:
I have found a workaround.
After doing a successful upgrade using another Windows 8.1 Pro OEM key, I go to System, click "Change Product Key" and enter the MAK key which is then accepted.

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Jackie Man IT ManagerCommented:
Just go to the same MS licensing portal where you get the MAK key to download the corresponding ISO file.

Then, follow the advise of McKnife for the inplace upgrade.

Alternatively, you can setup one computer using inplace upgrade, sysprep and capture the system image and deploy the image to the remaining computers by Windows ADK.

https://www.experts-exchange.com/articles/12635/Windows-8-and-8-1-Sysprep-Guide.html
Barry KaySystems EngineerAuthor Commented:
It works.
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