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
import org.elasticsearch.common.settings.Settings;
import org.elasticsearch.core.Strings;
import org.elasticsearch.core.TimeValue;
import org.elasticsearch.core.Tuple;
import org.elasticsearch.index.IndexMode;
import org.elasticsearch.index.IndexSettings;
import org.elasticsearch.index.MapperTestUtils;
import org.elasticsearch.test.ESTestCase;
import org.junit.Before;
import java.io.IOException;
import java.time.Instant;
import java.time.temporal.ChronoUnit;
import java.util.List;
import static org.elasticsearch.common.settings.Settings.builder;
import static org.elasticsearch.datastreams.DataStreamIndexSettingsProvider.FORMATTER;
import static org.hamcrest.Matchers.contains;
import static org.hamcrest.Matchers.containsInAnyOrder;
import static org.hamcrest.Matchers.equalTo;
public class DataStreamIndexSettingsProviderTests extends ESTestCase {
    DataStreamIndexSettingsProvider provider;
    @Before
    public void setup() {
        provider = new DataStreamIndexSettingsProvider(
            im -> MapperTestUtils.newMapperService(xContentRegistry(), createTempDir(), im.getSettings(), im.getIndex().getName())
        );
    }
    public void testGetAdditionalIndexSettings() throws Exception {
        Metadata metadata = Metadata.EMPTY_METADATA;
        String dataStreamName = "logs-app1";
        Instant now = Instant.now().truncatedTo(ChronoUnit.SECONDS);