Depends on your version of Windows. Windows 10 has been reported for be fairly successful in moving from one system to another without needing to reinstall. All previous versions of Windows, I would definitely recommend a Windows resinstall. As for OEM vs Retail versions of Windows, if you are running the Windows 10 Anniversary update and you use your Microsoft account to log into the PC, then you do not need a new Windows license. Any other OEM version of windows will require reactivation, and depending on Microsoft, maybe a new license.
-Wolf sends