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/main/java/org/elasticsearch/datastreams/DataStreamIndexSettingsProvider.java
import org.elasticsearch.cluster.metadata.Metadata;
import org.elasticsearch.common.UUIDs;
import org.elasticsearch.common.compress.CompressedXContent;
import org.elasticsearch.common.settings.Settings;
import org.elasticsearch.common.time.DateFormatter;
import org.elasticsearch.core.CheckedFunction;
import org.elasticsearch.core.TimeValue;
import org.elasticsearch.index.IndexMode;
import org.elasticsearch.index.IndexSettingProvider;
import org.elasticsearch.index.IndexSettings;
import org.elasticsearch.index.mapper.DateFieldMapper;
import org.elasticsearch.index.mapper.KeywordFieldMapper;
import org.elasticsearch.index.mapper.Mapper;
import org.elasticsearch.index.mapper.MapperBuilderContext;
import org.elasticsearch.index.mapper.MapperService;
import org.elasticsearch.index.mapper.MappingParserContext;
import java.io.IOException;
import java.io.UncheckedIOException;
import java.time.Instant;
import java.time.temporal.ChronoUnit;
import java.util.ArrayList;
import java.util.List;
import java.util.Locale;
import static org.elasticsearch.cluster.metadata.IndexMetadata.INDEX_ROUTING_PATH;
/**
 * An {@link IndexSettingProvider} implementation that adds the index.time_series.start_time,
 * index.time_series.end_time and index.routing_path index settings to backing indices of
 * data streams in time series index mode.
 */
public class DataStreamIndexSettingsProvider implements IndexSettingProvider {