Currently it is possible to migrate an existing data stream to a time series data stream.
This is done by updating the index template that created the data stream. 
Then after a rollover the data stream is a time series data stream. 

Via a similar way it should be possible to change time series data stream back 
to a regular data stream. Currently when changing the `index.mode` setting
on an index template from `time_series` to `standard` (or removing the setting)
results in a validation error.
server/src/main/java/org/elasticsearch/cluster/metadata/DataStream.java
}
    public List<Index> getIndices() {
        return indices;
    }
    public long getGeneration() {
        return generation;
    }
    public Index getWriteIndex() {
        return indices.get(indices.size() - 1);
    }
    /**
     * @param timestamp The timestamp used to select a backing index based on its start and end time.
     * @param metadata  The metadata that is used to fetch the start and end times for backing indices of this data stream.
     * @return a backing index with a start time that is greater or equal to the provided timestamp and
     *         an end time that is less than the provided timestamp. Otherwise <code>null</code> is returned.
     */
    public Index selectTimeSeriesWriteIndex(Instant timestamp, Metadata metadata) {
        for (int i = indices.size() - 1; i >= 0; i--) {
            Index index = indices.get(i);
            IndexMetadata im = metadata.index(index);
            
            
            if (im.getIndexMode() != IndexMode.TIME_SERIES) {
                
                
                continue;
            }
            Instant start = im.getTimeSeriesStart();
            Instant end = im.getTimeSeriesEnd();