How to log as admin after RDS grace period expires?

I have a remote machine with Windows Server 2008 R2, on which I installed the RDS Host Services. I was installing the CALs a few days ago, but unfortunately the grace period expired before I finished the process. I had 5 CALs on hand and needed to purchase another 5 CALs for the rest of the users.

Now I cannot access the server anymore. I tried to login as Administrator using "mstsc /admin", but it doesn't work. I have access to the filesystem of the server, because I can mount it on another system. But I cannot login thru the Remote Desktop Connection. And I don't have physical access to the server.

How can I login and finish the RDS License setup? Or what should I change on the filesystem so I can reboot the machine, login and finish the RDS setup?

Thanks.