Drupal updating form fields on file upload
After many hours, I've found a solution!  Thanks goes to Matthew Davidson, who posted this workaround on drupal.org, full post The working code looks like this:

function mymodule_form_alter(&amp;$form, &amp;$form_state, $form_id){
   if ($form_id == 'mymodule_node_form'){

       // create a button to populate the form
       $form['mymodule_pop_btn'] = array(
           '#type' =&gt; 'submit'.
           '#value'=&gt;t('Populate Fields'),
           '#submit'=&gt;array('mymodule_populate_fields'),
       );
    }
}

//function to populate the form fields
function mymodule_populate_fields($form, &amp;$form_fields){
    $form_state['input']['my_field']['und'][0]['value'] = 'Some New Value';
    $form_state['rebuild'] = TRUE;
    $form_state['no-cache'] = TRUE;
}