thanks Jeff...
BTW, I found this FAQ below, seems like if we upgrade from Win10 TP, we will get the non-genuine license of Win 10, but probably it doesn't matter because of this 12 months free upgrade. Otherwise we need to go back first to our original SP3 Win8.1
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Will I be able to upgrade from the Windows 10 Technical Preview to the final release?
There is has been some debate regarding qualification for the final release of Windows 10.
Microsoft has noted the intention is to provide a direct upgrade path from the Preview to the final release. Of course, decisions can change. Recently, the company noted that you could upgrade non genuine licenses to Windows 10, but later it was found out, that license would be non genuine.
So, to be on the safe side, its best you ensure you have a valid genuine Windows 7 or Windows 8.1 license installed in order to qualify for the final release.
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This is referring to non-genuine editions of Windows 7 and 8. The Windows 10 license you have in the insider program is considered a "genuine license"