What should you do to ensure that the RDS Per Device CALs allocated to the old client computers …?

Your network contains two servers that run Windows Server 2008 R2.
The servers are configured as shown in the following table.

The network contains 100 client computers that connect to Remote Desktop Services (RDS) on Server2.
Server1 has 100 Remote Desktop Services Per Device client access licenses (RDS Per Device CALs) installed.
You exchange 10 client computers for 10 new client computers.
You need to ensure that the RDS Per Device CALs allocated to the old client computers can be immediately reallocated to the new client computers.

What should you do?

A.
From the Remote Desktop Session Host Configuration console on Server2, modify the Licensing settings.

B.
From the Remote Desktop Licensing Manager tool on Server1, run the Manage RDS CALs wizard and click the Migrate action.

C.
From the Remote Desktop Licensing Manager tool on Server1, navigate to the Windows Server 2008 R2 – Installed RDS Per Device CALs node and run the Install Licenses wizard.

D.
From the Remote Desktop Licensing Manager tool on Server1, navigate to the Windows Server 2008 R2 – Installed RDS Per Device CALs node and click the Revoke RDS CAL action.

Explanation:
Revoke a Remote Desktop Services Per Device Client Access License
When a Remote Desktop Session Host (RD Session Host) server is configured to use Per Device licensing mode, and a client computer or device connects to an RD Session Host server for the first time, the client computer or device is issued a temporary license by default. When a client computer or device connects to an RD Session Host server for the second time, if the Remote Desktop license server is activated and enough RDS Per Device CALs are available, the license server issues the client computer or device a permanent RDS Per Device CAL. If the license server is not activated or does not have any RDS Per Device CALs available, the device continues to use the temporary license. The temporary license is valid for 90 days.

In some circumstances, you might want or need to return an RDS Per Device CAL that has been issued back to the available pool on the license server before the automatic expiration period has been reached. For example, you might want to do this if the client computer or device is no longer a part of your environment.
You can revoke an RDS Per Device CAL by using the Remote Desktop Licensing Manager tool.

After you have revoked the RDS Per Device CAL, that RDS Per Device CAL is immediately available to be issued to another client computer or device. Revocation is not a substitute for ensuring that you have enough RDS Per Device CALs to support your environment.
You can only revoke up to twenty percent of the number of RDS Per Device CALs of a particular version installed on your license server.

Source: http://technet.microsoft.com/en-us/library/cc732416.aspx