Windows 10: OEM key linked to my Microsoft account does not allow me to upgrade legitimately

Hello, I have a question. In the past, I bought a Windows 10 Home directly from Microsoft. However, a few years later, I purchased a Windows 10 Pro OEM from an alternative website. I regret having bought the OEM illegally, and I would like to upgrade the Home key to Pro directly through Microsoft. Today, I am using Windows 10 Home, and when I go to the Microsoft Store to update the key to the Pro version, it shows the option to install as if I already had an original key, even though it is an OEM key that was previously used on an older computer of mine.

:)