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.
modules/data-streams/src/javaRestTest/java/org/elasticsearch/datastreams/TsdbDataStreamRestIT.java
}
    private static Map<?, ?> getIndex(String indexName) throws IOException {
        var getIndexRequest = new Request("GET", "/" + indexName + "?human");
        var response = client().performRequest(getIndexRequest);
        assertOK(response);
        return entityAsMap(response);
    }
    private static Instant getStartTime(Map<?, ?> getIndexResponse) throws IOException {
        assert getIndexResponse.keySet().size() == 1;
        String topLevelKey = (String) getIndexResponse.keySet().iterator().next();
        String val = ObjectPath.evaluate(getIndexResponse.get(topLevelKey), "settings.index.time_series.start_time");
        return Instant.from(DateFormatter.forPattern(FormatNames.STRICT_DATE_OPTIONAL_TIME.getName()).parse(val));
    }
    private static Instant getEndTime(Map<?, ?> getIndexResponse) throws IOException {
        assert getIndexResponse.keySet().size() == 1;
        String topLevelKey = (String) getIndexResponse.keySet().iterator().next();
        String val = ObjectPath.evaluate(getIndexResponse.get(topLevelKey), "settings.index.time_series.end_time");
        return Instant.from(DateFormatter.forPattern(FormatNames.STRICT_DATE_OPTIONAL_TIME.getName()).parse(val));
    }
    static String formatInstant(Instant instant) {
        return DateFormatter.forPattern(FormatNames.STRICT_DATE_OPTIONAL_TIME.getName()).format(instant);
    }
    static String formatInstantNanos(Instant instant) {
        return DateFormatter.forPattern(FormatNames.STRICT_DATE_OPTIONAL_TIME_NANOS.getName()).format(instant);
    }
    static Instant parseInstant(String input) {
        return Instant.from(DateFormatter.forPattern(FormatNames.STRICT_DATE_OPTIONAL_TIME.getName()).parse(input));
    }
}