Change power head
You need a voltmeter to determine which wire of the pigtail is "+" and which is "-". Then you need to look on the case of the equipment that the new jack fits into and determine whether the outside shell is supposed to be "+" or "-".
Use the ohmeter function to determine which pin of your new connector is the shell and you have all the info you need.
Note, the new plug has two pins on the back because it was meant to plug into a matching receptacle that allowed you to plug in the connector two different ways. This allows the user to hookup the pigtail wires in either order and then just reverse the plug if the order is wrong.