No, you shouldn't buy a new licence for your McAfee product. You will need to reinstall it by using your account after upgrading Windows. If you just upgrade your operating system without doing a clean install, you should first remove all McAfee software using Revo Uninstaller, so you won't have any leftovers for possible future conflicts. Then you should install the new operating system. Next, download and run a patch from McAfee for clearing temporary files and cookies:
Download and install the software by utilizing your account. If you encounter any problems you can always check the detailed steps of the procedure by following McAfee's tutorial.