If you upgrade to Windows 10, either with free upgrade or you got a new system that comes with the latest Windows version, you would probably get the Windows 10 Home. However, if you have the pro version of Windows 7 or Windows 8.1 before you upgrade, you would get Windows 10 Pro.
In case you want to upgrade to windows 10 pro from Windows 10 Home, Microsoft has made this process easy for you, you can do this upgrade by just using a default product key officially offer by Microsoft itself.
In a thread tagged How to upgrade 1511 from Home to Pro on Microsoft’s Community Forum a question was asked “I’m not having any luck figuring out how to upgrade from Home to Pro with an existing key, Anyone know how to force an install of Pro when the installer automatically installs Home with no option given to change it?”
Here is a reply Charles of Microsoft gave;
From your Windows 10 Home running Version 1511, enter the Windows 10 Pro Default key under change product key.
This default key will not activate the system, just take you to Pro so you can activate using a valid Pro key that you will provide.
Once you have done this, the system will go through an upgrade process from Home to Pro, but will not be activated on Pro.
upgrade to windows 10 pro from Windows 10 Home
All you have to do is switch product keys, follow these steps carefully
Open Settings and go to Update & security
Under update & security page, scroll down and select Activation > Change Product Key.
Now enter the product key provided by Microsoft “VK7JG-NPHTM-C97JM-9MPGT-3V66T“. Wait as the upgrade process takes time to complete.
Mind you, this key will not activate the pro version for you, it’s just to upgrade your current version of Windows to Windows 10 Pro.
To buy a key for Windows 10 Pro, go to Settings > update & security > Activation and click the Go to Store.
You can as well use your Windows 8 Pro key to activate it or simply buy a product license key and enter it to get it activated.
Hope it helps.