给text字段添加了个 keyword，那么旧数据怎么给keyword初始化，环境是es5.5

PUT user_index/user_type/_mapping

{

    "properties": {

        "userNickName": {

            "type": "text",

            "fields": {

                "keyword": {

                    "type": "keyword",

                    "ignore_above": 256

                }

            }

        }

    }

}