I'm just worried about the product key situation being a bitch(to clarify I am a Windows 8 product key and I've upgraded up over the years)
If you installed Windows 10 on top of another version of Windows *and* verified that Windows 10 managed to activate itself, you should be fine - after you install Windows 10 on the same hardware again it will recognize the machine and activate itself.
Worst case, if Windows 10 didn't manage to get to activation yet after you installed it on top of the previous version of Windows, because the machine went dead before that, you will have to reinstall the previous version of Windows, activate it via the key that you have, then install Windows 10 on top, wait for it to activate (or force it to do it) - and after that you will be able to reinstall Windows 10 on that machine without worrying about activation.