We have an existing index with data and mapping with a date field with two existing formats, e.g.:

existing mapping:
"tzoffset_timestamp": {
"type": "date",
"store": true,
"format": "EEE MMM dd HH:mm:ss.S Z yyyy||EEE MMM dd HH:mm:ss.SSS Z yyyy",
"include_in_all": true
}

when executing curl put mapping command with an additional format:

"tzoffset_timestamp": {
    "type": "date",
    "store": true,
    "format": "EEE MMM dd HH:mm:ss.S Z yyyy||EEE MMM dd HH:mm:ss.SSS Z yyyy||yyyy-MM-dd'T'HH:mm:ss.SSSZZ",
    "include_in_all": true
}

succeeds with "ok / acknowledged", yet getting the mapping back provides original two formats.