The soap request for changing the power policy is in Intel_AMT_SDK_6.0_Hot_Fix_2450\DOCS\WSDL\SecurityAdministrationInterface.wsdl
The command youre looking for is SetActivePowerPackage()
While it is possible to set the power package using soap, Wsman is the preferred method moving forwardsince the soap interface has been deprecated and will be removed in future versions of the platform.
You can change the powerspolicyusing the CIM class AMT_SystemPowerScheme. SetPowerScheme() in Wsman