Comment To activate, you MUST upgrade not clean install (Score 1) 187
Turns out that to activate, you MUST do the upgrade from an activated Windows 7/8/8.1. Apparently that will register the hardware signature for the activated Windows 10 on Microsoft's servers. After you've upgraded to Windows 10 that way, you can then go back and do a clean install if desired - because that hardware signature is known it'll activate on its own after the clean install, or at least that's how I found it described.
I was just glad that I had the Windows 7 Pro SP1 64-bit ISO handy - I ended up reinstalling that, activating (had to call in, when they offer you the smartphone option, TAKE IT), never even installing updates, then installing the upgrade immediately.