First of all, please use English in MDL. If you don't speak, just use Google Translate. Believe me, my English is not so good either. You have to follow traditional upgrade method (in your case from Windows 8.1) to Windows 10 once. After you successfully upgrade your OS to Windows 10 via Windows Update, your current license key will be replace with brand new Windows 10 license and this will be permanently yours. After this step, feel free to made a clean install your system via DVD or USB anytime. Your new license key will tied up to your hardware and save in Microsoft servers. You're not gonna loose your license key.