If a Windows 10 Insider Preview user need to re-install from scratch the final windows 10 pro on the same computer what serial need to use? Are the Insider Preview credentials logon enough? I cannot find info on this case online.
You will need to buy a license. Only windows 7 and windows 8 users are eligible to apply for the free upgrade.
If you have an old windows 7 or 8 license, install that windows first, then use the upgrade function to register your hardware ID. This should allow you to install windows 10 in the future without a problem.