Multistep ajax form, rebuild form and replace contents with the current step
You dont' have to call drupal_get_form explicitly, instead mark form to rebuild.

In your ajax powerd element define two attributes: #submit, and #ajax. 

#submit references the function that makes the action related to the submission  (in your case changing the step value to 2 in form state). Also, set $form_state['rebuild'] to TRUE.

#ajax, where you set the ajax parameters and the PHP function that will return the result form the built form (in this case, the rebuilt one). When this fucntion is called form is build (or rebuilt) and you only have to return the form part you need.

Take into account that in the #ajax attribute you can set the HTMl element that will be replaced by the answer, so you don't need to use ajax commands.

&lt;?php

  funtion my_form_builder($form, &amp;$form_state) {
     if (empty($form_state['step'])) {
      $form_state['step'] = 'step_1';
     }
     $step = &amp; $form_state['step'];
     // build step 1
     if ($step == 'step_1') {
       $form['first_submit'] = array(
         '#type' =&gt; 'submit',
         '#ajax' =&gt; array(
           'callback' =&gt; 'form_first_ajax_callback',
          ),
         '#executes_submit_callback' =&gt; TRUE,
         '#submit' =&gt; array('my_form_builder_callback'),
       );
     }
     // build step 2
     if ($step == 'step_2') {
       $form['elem_2'] = array(...)
       $form['elem_2_1'] = array(...)
     }
     return $form;
  }

  function my_form_builder_callback($form, &amp;$form_state) {
     $current_step = &amp; $form_state['step'];
     // store values...
     // move on to the next step
     $current_step = 'step_2';
     $form_state['rebuild'] = TRUE;
  }


  function form_first_ajax_callback($form, &amp;$form_state) {

  $commands[] = ajax_command_replace('#main-content', render(form['element_to_return']));
  return array(
   '#type' =&gt; 'ajax',
   '#commands' =&gt; $commands,
  );
  }


Example uses the ajax commands aproach.

Check the Form API internal workflow illustration.