The ISO won't have a key assigned. The machine running Windows 10 will be authorised once you've installed from the ISO, supplied a valid Windows 7/8 key (that hasn't already been used to upgrade to Windows 10) and then activated online. You won't need to supply the Windows 7 key again if you wanted to do a fresh install further down the line.
I recently upgraded a laptop from Windows 7 to Windows 10. A week later the motherboard died. I swapped out the motherboard and then Windows 10 booted fine but then reported that it wasn't activated. After about half an hour on live chat to Microsoft, they had re-activated Windows 10 so it was associated with the new motherboard.