JavaScript: Change target then change selected option

Hi JavaScript experts,
 
I’ve been struggling with this for many hours, and would appreciate your help.  Here's a majorly simplified version of my code.
 
I’ve got a form in HTML5 with these controls:
    Form: <form method=post name=mainForm id=mainForm>  (I’m not bothering to specify the action because the script calls itself regardless of how the form is submitted.)
    Search: <select type=submit name=search value=Search>
    Go: <select type=submit name=go value=Go>
    Add: <select type=submit name=add value=Add>
    Clear: <button type=submit formaction="">Clear</button>
    Destination:
      <select name=destination id=destination>
          <option value=original_window id=current_window>Current Window</option>
          <option value=new_window>New Window</option>
          <option value=csv>CSV File</option>
          <option value=xlsx>XLSX File</option>
      </select>
  Plus a lot of text fields, etc.
 
When someone clicks Search, if the selected Destination is “New Window”, then I want the form’s target to change to “’_blank’” (instead of the default) so the search results go to a new window, and then I want the Destination to be changed back to “Original Window”.  (That's the only scenario where I want a "New Window" to appear.)  I (almost) achieved that with this HTML:

<input type=submit name=search value=Search onclick="changeTarget()">

And this JavaScript:

function changeTarget() {
  if (document.getElementById('format').value == 'new_window') {
    document.getElementById('mainForm').target = '_blank';
    // Not sure of the significance of using either or both of the following 2 lines, but that’s not my main problem.
    document.getElementById('destination').value = 'original_window';
    document.getElementById('original_window').selected = true;
  }
  else {
    document.getElementById('mainForm').target = '_self';
  }
}

When I run the above code and do a Search with a Destination of “New Window”, a new window is opened (that's good), and the Destination option gets set back to “Original Window” in the original window (that's good), but presumably because of the latter, the “destination” HTTP parameters get passed with a value of “original_window”, presumably because the Destination was reverted before the new window was created.

Questions:
Q1. Any ideas how to avoid the above behaviour, i.e. so a HTTP parameter value of “new_window” is passed for the “destination” attribute?

Q2. And as a minor 2nd part to this, what is the difference between doing this:
    document.getElementById('destination').value = 'original_window';
and this:
    document.getElementById('original_window').selected = true;
From my tests, either one seems to have the same result in my case, so I don't think both are necessary.

I would have liked to put this in a jsfiddle.net for you to test, but I don’t think it’s going to work because of the change of form target.
No jQuery solutions, please - just native JavaScript, (if JavaScript is required at all).

Thanks.
Tel2