How to change gazebo plugin parameter

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I am using Hydro on Ubuntu 12.04. When I spawn create base turtlebot, turtlebot begins to navigate on its own. I suspect that in file create_gazebo.urdf.xacro under create_description package like in this code snippet alwaysOn parameter is causing this. Is there any way to change value of this parameter in launch file or only way is changing this file ?

....
<robot xmlns:xacro="http://ros.org/wiki/xacro" name="turtlebot_gazebo">
<xacro:macro name="sim_create">
  <gazebo>
    <plugin name="create_controller" filename="libgazebo_ros_create.so">
      <alwaysOn>true</alwaysOn>
      <node_namespace>turtlebot_node</node_namespace>
      <left_wheel_joint>left_wheel_joint</left_wheel_joint>
      <right_wheel_joint>right_wheel_joint</right_wheel_joint>
      <front_castor_joint>front_castor_joint</front_castor_joint>
      <rear_castor_joint>rear_castor_joint</rear_castor_joint>
      <wheel_separation>.260</wheel_separation>
      <wheel_diameter>0.066</wheel_diameter>
      <base_geom>base_footprint_collision_base_link</base_geom>
      <updateRate>40</updateRate>
      <torque>1.0</torque>
    </plugin>
  </gazebo>
.....