The How-To Geek posted an excellent overview of slmgr, a tools used to modify and control Windows Licensing:
Windows activation is designed to be as foolproof as possible, so Microsoft’s graphical tools keep it simple. If you want to do something more advanced like remove a product key, force an online activation, or extend the activation timer, you’ll need Slmgr.vbs.
Source: How to Use Slmgr to Change, Remove, or Extend Your Windows License
What do you do, however, if Windows still comes up “Not Genuine” and will no longer rearm? Using a KMS server, this just shouldn’t happen, but in reality it occurs all the time. In the past, we have taken to wiping the machine and starting over, but recently I actually had two recently imaged machines do this!
I found out that in these two cases, I could actually use System Restore to take it back to a previous restore point before things went wrong! Please note that this will not always work, and I really did not expect it to work this time. The only conclusion I can come to is that an update did not properly apply and cause the issue to begin with.
At any rate, it might save someone some time.
I’ve actually had dreams (nightmares?) lately about Microsoft license activation, believe it or not, as it is such a pain in the ***. Both Microsoft Windows and Microsoft Office can get to the point where they cannot or will not talk to the KMS server and grab a license for one reason or the other. So, this is how to force the issue when you have enterprise versions of this software.
One scenario is where Windows quits talking to the KMS server, goes beyond the grace period and then declares that this copy of windows is not genuine. You can try to activate it, but it will refuse because it is out of the grace period. Of course, it is outside of the grace period because it did not activate, so this causes a bit of a catch-22.
The way to break out of the cycle is to extend the grace period. You do this via the “rearm” command. For example, open a command prompt with elevated privileges and type:
That will rearm activation, thus extending the grace period, and then activate windows (it assumes KMS is online).