So the potential is there that MS could blacklist keys that are on systems that have upgraded to Windows 10. I don't think they will ever do such a thing and I wouldn't get worried about it.
This is my main doubt while, eventually, making a rollback from Win10 to the original OS (for example, back to Win 7 Home Premium).
Besides being blaclisted, the upgrade could make your system no more eligible for the re-activation of the previous operative system !!!
At this point, you'd be locked in a "cul-de-sac", and be forced only to use the new Win10y.
A very unpleasant situation/condition, not only for the user himself but, mainly, with reference to Microsoft's reputation !!!