The Scalability Server an agent reports to can be changed via command line or via a configuration policy.
If you were to change the reporting Scalability Server of a machine you could do so either by running a command line on the agent machine or by changing the parameter 'Common Agent--> Common' in the configuration policy on the domain manager
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Important Note: If the Scalability Server the agent was reporting to, is no longer available then the configuration policy method will not work. You will have to use the command line method (using a script/login script) to re-direct the agents to a new Scalability Server.