You need to re-register some client machines to a different Task Server and you know this can be done with the "Reset Agent" button under the "Task Status" tab on the Symantec Management Agent UI as seen here:
Can this be done remotely from the SMP Console when you aren't able to access a managed machine?
ITMS 8.x
There is a default task that can be used to accomplish this "Reset Agent" functionality. See the Reset Task Agent Page.
There is a default task that can be used send a "Reset Task Agent" to managed machines that will do this. You can create a new "Client Job" and add a "New" task using "Reset Task Agent" as seen here:
Just make sure you have "Check NS for an updated site server list" selected as shown above so the managed machine can re-evaluate the available Task Servers.
Also, if you want to know which computers are registered to a specific Task Server, there is a report that you can use that shows the Task Server(s) and how many clients each has registered. If you double-click on the "client count", it will drill-down to a list of machines. This is found in the SMP Console:
Reports>Task Server>Status>Task Server Summary
Note: Here is VBS script to do it if you want to create your own script or process:
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dim objClient
set objClient = CreateObject("Altiris.AeXClient")
dim objCTA
set objCTA = objClient.HostedObject("Altiris.ClientTaskAgent")
objCTA.ResetAgent 1