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/test/java/org/elasticsearch/datastreams/DataStreamIndexSettingsProviderTests.java
Instant sixHoursAgo = Instant.now().minus(6, ChronoUnit.HOURS).truncatedTo(ChronoUnit.SECONDS);
        Instant currentEnd = sixHoursAgo.plusMillis(lookAheadTime.getMillis());
        Metadata metadata = DataStreamTestHelper.getClusterStateWithDataStream(
            dataStreamName,
            List.of(new Tuple<>(sixHoursAgo, currentEnd))
        ).getMetadata();
        Instant now = sixHoursAgo.plus(6, ChronoUnit.HOURS);
        Settings settings = Settings.EMPTY;
        var result = provider.getAdditionalIndexSettings(
            DataStream.getDefaultBackingIndexName(dataStreamName, 1),
            dataStreamName,
            true,
            metadata,
            now,
            settings,
            List.of(new CompressedXContent("{}"))
        );
        assertThat(result.size(), equalTo(2));
        assertThat(result.get(IndexSettings.TIME_SERIES_START_TIME.getKey()), equalTo(FORMATTER.format(currentEnd)));
        assertThat(
            result.get(IndexSettings.TIME_SERIES_END_TIME.getKey()),
            equalTo(FORMATTER.format(now.plusMillis(lookAheadTime.getMillis())))
        );
    }
    public void testGetAdditionalIndexSettingsDataStreamAlreadyCreatedTimeSettingsMissing() {
        String dataStreamName = "logs-app1";
        Instant twoHoursAgo = Instant.now().minus(4, ChronoUnit.HOURS).truncatedTo(ChronoUnit.MILLIS);
        Metadata.Builder mb = Metadata.builder(
            DataStreamTestHelper.getClusterStateWithDataStreams(
                List.of(Tuple.tuple(dataStreamName, 1)),