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I assume you mean changing the Application Name from the Controller UI by going to "Edit Application Properties".
If so, you can do so and no JVM/agent restart is required at that point in time, however, and this is important, you do need to go and update all of the agent's configuration where you have specified the application name to match this new name before next time the JVMs/Agents do get restarted. Otherwise, they will start reporting into a "new" application with same name as the old one once they restart.