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/MetadataIndexTemplateServiceTests.java
import org.elasticsearch.common.settings.Settings;
import org.elasticsearch.core.Tuple;
import org.elasticsearch.env.Environment;
import org.elasticsearch.index.IndexSettingProviders;
import org.elasticsearch.indices.EmptySystemIndices;
import org.elasticsearch.indices.IndicesService;
import org.elasticsearch.indices.InvalidIndexTemplateException;
import org.elasticsearch.indices.ShardLimitValidator;
import org.elasticsearch.test.ESSingleNodeTestCase;
import java.time.Instant;
import java.time.temporal.ChronoUnit;
import java.util.Collections;
import java.util.List;
import java.util.Set;
import static org.elasticsearch.cluster.metadata.DataStreamTestHelper.generateTsdbMapping;
import static org.elasticsearch.common.settings.Settings.builder;
import static org.elasticsearch.datastreams.MetadataDataStreamRolloverServiceTests.createSettingsProvider;
import static org.elasticsearch.indices.ShardLimitValidator.SETTING_CLUSTER_MAX_SHARDS_PER_NODE;
import static org.hamcrest.Matchers.containsString;
import static org.hamcrest.Matchers.equalTo;
import static org.mockito.Mockito.mock;
import static org.mockito.Mockito.when;
/**
 * Variant of MetadataIndexTemplateServiceTests in server module, but with {@link DataStreamIndexSettingsProvider}.
 */
public class MetadataIndexTemplateServiceTests extends ESSingleNodeTestCase {
    public void testValidateTsdbDataStreamsReferringTsdbTemplate() throws Exception {
        var state = ClusterState.EMPTY_STATE;
        final var service = getMetadataIndexTemplateService();
        var template = new ComposableIndexTemplate(