This has been debated in numerous threads so instead or replying in all those I started this one. Some of you have noticed like me that you got a generic/common product code. Future activation on a clean install for the free upgrade is tied to the hardware and your Microsoft Account, not the product code. From what I'm reading you don't even bother entering one on a clean install. If you don't have a unique product code to transfer to another PC your not transferring Windows. Not that install anyway. Not the free upgrade. Free for the life of the device.
Microsoft posted new keys for Windows 10 on my MSDN account. They are listed as Retail keys. I would use them to install on new hardware. Those keys would be transferable. I assume if you buy Windows 10 Retail via Newegg etc, you'll get a product code in the box, and that version will be transferable. Just like in the past.
To me it looks like the Free upgrade is not transferable. Which is what I thought it would be all along.