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
if (timestamp.compareTo(start) >= 0 && timestamp.compareTo(end) < 0) {
                return index;
            }
        }
        return null;
    }
    /**
     * Validates this data stream. If this is a time series data stream then this method validates that temporal range
     * of backing indices (defined by index.time_series.start_time and index.time_series.end_time) do not overlap with each other.
     *
     * @param imSupplier Function that supplies {@link IndexMetadata} instances based on the provided index name
     */
    public void validate(Function<String, IndexMetadata> imSupplier) {
        if (indexMode == IndexMode.TIME_SERIES) {
            
            var startAndEndTimes = indices.stream().map(index -> {
                IndexMetadata im = imSupplier.apply(index.getName());
                if (im == null) {
                    throw new IllegalStateException("index [" + index.getName() + "] is not found in the index metadata supplier");
                }
                return im;
            })
                .filter(
                    
                    im -> im.getTimeSeriesStart() != null && im.getTimeSeriesEnd() != null
                )
                .map(im -> {
                    Instant start = im.getTimeSeriesStart();
                    Instant end = im.getTimeSeriesEnd();
                    assert end.isAfter(start);