Using _reindex to convert a string field to a float

The subject says it all. I have a field that is intermittently being saved in an index as:

{
  "index_with_string" : {
    "mappings" : {
      "timeElapsed" : {
        "full_name" : "timeElapsed",
        "mapping" : {
          "timeElapsed" : {
            "type" : "text",
            "fields" : {
              "keyword" : {
                "type" : "keyword",
                "ignore_above" : 256
              }
            }
          }
        }
      }
    }
  }
}

I would like mapped thus:

{
  "index_with_float" : {
   "mappings" : {
      "timeElapsed" : {
        "full_name" : "timeElapsed",
        "mapping" : {
          "timeElapsed" : {
            "type" : "float"
          }
        }
      }
    }
  }
}

I tried the following convert pipeline:

{
  "description": "converts the field timeElapsed to a float from a string",
  "processors" : [
    {
      "convert" : {
        "field" : "timeElapsed",
        "type": "float"
      }
    }
  ]
}

But I got invalid mapping errors with the error: "field [timeElapsed] not present as part of path [timeElapsed]" when I ran _reindex.

Can anyone help me with the correct convert pipeline?

Thank you.