Remove or increase ignore_above on a text.keyword field

I am trying to increase or remove the ignore_above on a particular field on my index:

      "emlBody" : {
        "type" : "text",
        "fields" : {
          "keyword" : {
            "type" : "keyword",
            "ignore_above" : 256
          }
        },
        "fielddata" : true
      },

I can find any solution for this after a round of Googling. I am trying have emails indexed so I can ran a scripted field on the text and extract a value with regex. From my understanding, I need to run my painless script on a var.keyword field. But, my current index is discarding email content above 256

thanks

I think this the "You can't change the mapping of an index". You would have to create a new index with the desired mapping change and reindex.