node_update.php

<?php 
$nodes = node_load_multiple(NULL, array("type" => "webform"));
foreach ($nodes as $node) {
  $save_node = new stdClass();
  $save_node->nid = $node->nid;
  $save_node->vid = $node->vid;
  $save_node->type = $node->type;
  $save_node->field_slsa_category = $node->field_slsa_category_1;
  entity_get_controller("node")->resetCache(array($node->nid));
  field_attach_presave("node", $save_node);
  field_attach_update("node", $save_node);
}
if ($field_instance = field_info_instance("node", "field_slsa_category_1", "webform")) {
  field_delete_instance($field_instance);
}