How to change form submit button value after form submission?
You can do this with two steps. The first is to check whether $form_state['values'] has been set, and change the value accordingly:

if(!isset($form_state['values'])) {
  $form['submit'] = array (
    '#type' =&gt; 'submit',
    '#value' =&gt; t('Submit'),
  );
}
else {
  $form['submit'] = array (
    '#type' =&gt; 'submit',
    '#value' =&gt; t('Submit Again'),
  );
}


The next step is in your submit function, where you have to tell Drupal to rebuild the form (rather than build it from scratch)

$form_state['rebuild'] = TRUE;