I followed the path described for the free Win 10 upgrade: I installed it as an upgrade over an installed, and activated, Windows 8.1. (I found that it did not activate unless I chose to "keep everything".) After that, I did a clean install from a USB drive, deleting the partitions from the boot drive in the process. I skipped entering a key. The resulting Win 10 activated online.
I didn't try using the key from the original Win 10 installation. I did that in the past with one of the later preview builds, and the key wasn't accepted for installation, much less activation.
If someone cold tell me that the Win 10 key from an installation created by the free upgrade is anything more than a generic placeholder, the same for everyone, I'd be pleased to know it.