We're all making it far more complicated than it actually is. All you need to know is this:
1. You can't activate W10 without a valid W10 license key!
2. You can acquire a valid W10 license in one of two ways:
a) purchase a W10 license key online or through retail
b) upgrade your older W7 or W8.x license key to a W10 license key
2b is the tricky one! Doing this requires that you start with a genuine and activated W7 or W8.1 installation and then go through the online in-place upgrade process at least once! It doesn't matter if this happened before the 29th with an insider preview version of Windows, or after with the RTM version. It just must have occurred. This creates a new W10 license and associates it with both your hardware and your MS account. All that is stored in the cloud.
Without having gone through that process, your MS account has no record of your hardware being associated with a W10 license. If you do a clean install, Windows will ask you for a key during installation. All you'll have is your older license key and that won't be accepted as a valid substitute for a W10 license.