It depends on how you apply the Preview.
If you install 10 Preview over your 7/8.1 install using the Windows Update method:
Yes, you will be able to download the Final release of Windows 10 when its launched an turn your Preview 10 into licensed 10 final, upgrading your 7/8.1 key.
If you wipe your PC and install 10 Preview clean from an ISO:
No, you cannot turn this into a 'properly' licensed 10 Final. However, you will get the 10 Preview upgraded to the code of 10 Final as just another Preview build sent to you. This is not 'properly' licenced, and instead is licensed under the Insider Preview Program. This will be treated like any other Pre Release build and have an expirey. To keep Windows working you will need to keep taking the new Preview builds as they are released. If you opt out of the Insider Program, your install will stop updataing and will eventually expirey and stop working. You will then need to format and install a 'proper' licensed copy.
So, if you want to move to 10 Final on the 29th using a proper license, upgrading your 7/8.1 key, then the way to do it is to install the Technical Preview over you 7/8.1 build using the Windows Update method.
