Is there a way to allow entering a value in a node's field if inserting a new node, but make the field disabled if updating that same node?

function my_module_form_node_form_alter(&$form, &$form_state, $form_id) {
  $node = $form_state['node'];

  if ($form_id == "my_content_type_node_form") {

    global $user;

    if (isset($user)) {

      $user_has_standard_user_role = FALSE;

      // Get role information about the user.
      $user_roles = array();
      foreach ($user->roles as $key => $role) {
        $user_roles[$key] = $role;
      }

      foreach ($user_roles as $key => $user_role) {
        if ($user_role == "Standard User") {
          $user_has_standard_user_role = TRUE;
        }
      } // end iterate through user roles

      if ($user_has_standard_user_role) {

        // If this is a new node, let the user edit the field.
        if (!isset($node->nid) || isset($node->is_new)) {
          $form['field_status_field']['und']['#disabled'] = FALSE;
        }

        // If this is not a new node, then disable the field.
        else {
          $form['field_status_field']['und']['#disabled'] = TRUE;
        }
      } // end if user has Standard User role.

      // User is not Standard User, so disable the field.
      else { 
        $form['field_status_field']['und']['#disabled'] = TRUE;
      }
    } // end if user is set
  } // end if correct form ID
 } // end function