<input type="text" alt="abc">

function theme_form_alter(&$form, &$form_state, $form_id) {
    foreach($form["submitted"] as $newForm)
    {
        foreach($newForm as $keyid=>$frmArr)
        {
            if(isset($frmArr[]["#attributes"]["alt"]))
            {
                    $alt = $frmArr["#attributes"]["alt"];
                    unset($newForm[$keyid]["#attributes"]["alt"]);
                    $newForm[$keyid]["#attributes"]["data-alt"] = $alt;
            }
        }
    }
}

foreach($form["submitted"] as $newForm)
    {

        foreach($newForm as $keyid=>$frmArr)
        {
            if($keyid=='#attributes'){
                $newForm[$keyid]['data-alt']=$newForm[$keyid]['alt'] ;
                 unset($newForm[$keyid]['alt'] );
                $form["submitted"]['#attributes']['alt'] = array("sss");
                form_set_value($form["submitted"]['#attributes']['alt'], array("aaa"), $form_state);
            }
        }


    }

function act_form_alter(&$form, &$form_state, $form_id) {
  if ($form_id == 'the_id_of_your_form') {
    unset($form['submitted']['#attributes']['alt']);
    $form['submitted']['#attributes']['data-alt'] = 'abc';
  }
}