Hi, I have just upgraded my PC a few days ago and clean installed Win 8 pro.Now if I don't upgrade to win 8.1 and clean install Win 10 iso,when it releases, using a USB,will it activate itself using the valid 8 key?Or must I upgrade to win 8.1 prior to that?Please help 

• Edited by Snehanjan 14 hours 41 minutes ago

First you MUST upgrade to win 8.1 or you will not be offered win 10. 

Second.  There are two ways to upgrade to win 10 free.  Via windows upgrade, or via ISO.   I suggest ISO will be faster.

Whatever method you use to upgrade Nothing can/will be done until July 29 when win 10 is released

Win 10 FAQS http://www.microsoft.com/en-us/windows/windows-10-faq Win 10 features http://www.microsoft.com/en-us/windows/features
Win 10 Specs http://www.microsoft.com/en-us/windows/windows-10-specifications
How to upgrade http://www.microsoft.com/en-us/windows/windows-10-upgrade
Upgrade paths http://www.askvg.com/windows-10-upgrade-paths-and-things-to-know-before-upgrading/