Win10 preview upgrade problem

After the launch of the Windows 10 Technical Preview, many users have downloaded the latest system of the installation experience. However, there are users who have concerns. Now, directly install the Win10 technology preview version on Win8.1. Do you know if the official version can be overwritten directly after installation? Do you need to activate?
According to past experience, the preview version can be upgraded directly. The official version, after upgrading the official version, requires the activation key of Windows 10.
Explain the secret key mechanism of the Windows system. The secret key of each version can only activate this version. The activation key of Windows 8.1 is different from the KEY of Windows 10, and Windows cannot be activated by the activation key of Windows 8.1. 10. But in the future, Windows 8.1 will need to pay for the upgrade of Windows 10, and there is no relevant news for the time being. Even if you don't need to pay extra in the future, you need to install Windows 8.1 first and upgrade Windows 10.
With reference to the experience of Win8 release, after the technical preview, Windows 10 will also have a consumer preview or other test version. The official version of the launch may be in the second half of next year, when we can upgrade to the official version of Win10.