Changing a VSTS Agent's Controller

The controller pointed to by a Visual Studio Team Test Agent can be changed with AgentConfigUtil.exe. ie (from powershell):
. ${env:ProgramFiles(x86)}\*Test*Load*Agent\LoadTest\AgentConfigUtil.exe /controller:(machine)

You should then restart the Visual Studio Team Test Agent service:
net stop “Visual Studio Team Test Agent”
net start “Visual Studio Team Test Agent”