Last weekend I upgraded my computer as I have done during the last 14-15 years. I have changed mobo, CPU and memory.
For OS I have been running Windows 10 since August, upgraded from a W8.1, which I acquired directly from Microsoft store. At that time, a couple years ago, W8 cost me $40. It was not an OEM. The EULA mentions I could move the OS to another computer.
However for last week upgrade I wanted avoiding having problems with Windows 10 activation,so I decided to uninstall all drivers, turn off the computer, change parts (which involved a hard work cleaning the internal box) and start up. However Windows 10 started deactivated and refused to activate with the same product key. As others had that sort of problem and solved it by calling Microsoft, I did it so but MS refused as well to activate my W10.
What I had in front of me was the need to do a clean install, installing first W8. Fortunately I keep an spare hard disk which I used for the new installation. I did it so, then I installed W10 over W8 and finally I got activated. It is curious that the product key I got for the new activation was exactly the same I had in my previous computer.
I booted the old computer hard disk and it was again activated (with the same key), so go figure.
I decided to continue installing programs in the new install getting rid of many little unused programs I had before.
This kind of job takes time and care to get it done right.
If you are starting from zero instead of upgrading as my case, I would suggest you acquire anything that is not OEM. What I described above is what you will be doing next time you upgrade if you keep box, power source, drives and maybe graphics card as was my case.
And BTW, whatever you have heard, Windows 10 is faster and more reliable than W7 or W8.1.