I have a PC at the office, which was by default a Windows 8 Pro OEM licensed one (not 8.1), but for some unknown reasons, it was downgraded to Windows 7 Pro, which is again showing "OEM" in the activation Product ID.
I would like to format and perform a clean install of Windows 10 Pro (v. 1803) on the PC, and use the existing Windows 8 Pro OEM key to activate it after install.
I would like to get a second opinion before doing so, knowing that the PC did not receive any of the Windows 10 free upgrades etc. in the past?
Take a full disk image of the computer (I recommend Macrium Reflect) then try the WIndows 10 upgrade. Microsoft keeps extending the deadline. For example, last December I upgraded two machines to Windows 10 long after the official deadline. There is a good chance that the upgrade will go through.
After the upgrade, I recommend wiping the machine and doing a clean install of Windows 10. During the upgrade your computer will be registered with Microsoft and will thus be recognized as valid when you reinstall from scratch. The reason I recommend this second step is that upgrading from one OS version to another always leaves crap around from the previous OS.
While not a bad idea to do the second clean install the upgrade is smart enough to put all the "crap" in a folder called windows.old which has everything from the previous version which is why you are able to roll back. All you really have to do is delete that folder. Although the folder is called windows.old it is not just the windows os directory in there, it is everything from the old os.
It's not just files (Windows.old). It's also the registry. I prefer to start with a clean registry unpolluted by old stuff. I also find that my computer eventually suffers from "windows rot". Of course, I tend to install/uninstall software for evaluation quite frequently so that likely contributes. Generally, I keep benchmark images. When I start to feel the rot kicking in I reload the last benchmark image, apply all outstanding updates, them take a new benchmark. Of course, I also keep a change log that I update whenever I make a change so I have something to look back on to see what might have caused a problem, or to see what I might want to reinstall that went away on the last restore.
Having vms for software testing or whatever else is a great option. Once you have a clean install of an OS on VM, take a snapshot, then install whatever you want. You can instantly revert to the clean install and keep repeating the process. You can also save a snapshot of the os after you install your test software and then revert to clean again for some other software.
Using VM's you can keep your host OS clean forever.
I actually have a several vmware virtual machines configured for just that purpose. The small, potentially iffy stuff I install in a vm but the bigger, safe stuff I still prefer to test live. For example, earlier this spring I installed MicroAPL's APLX for Windows as well as Visual Studio 2017 (I had been plugging along with VS 2012 for some time) and when I get home in September I plan to install and evaluate Dyalog APL which bought out MicroAPL some months back. The Dialog version of APL downloads at about ten times the size for MicroAPL (about 250 meg compared to 21 meg).