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.
server/src/test/java/org/elasticsearch/cluster/metadata/DataStreamTests.java
import org.elasticsearch.cluster.ClusterState;
import org.elasticsearch.common.Strings;
import org.elasticsearch.common.UUIDs;
import org.elasticsearch.common.io.stream.Writeable;
import org.elasticsearch.common.settings.Settings;
import org.elasticsearch.core.Tuple;
import org.elasticsearch.index.Index;
import org.elasticsearch.index.IndexMode;
import org.elasticsearch.index.IndexSettings;
import org.elasticsearch.index.mapper.DateFieldMapper;
import org.elasticsearch.test.AbstractXContentSerializingTestCase;
import org.elasticsearch.xcontent.XContentParser;
import java.io.IOException;
import java.time.Instant;
import java.time.temporal.ChronoUnit;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.HashMap;
import java.util.HashSet;
import java.util.List;
import java.util.Locale;
import static org.elasticsearch.cluster.metadata.DataStreamTestHelper.newInstance;
import static org.hamcrest.Matchers.equalTo;
import static org.hamcrest.Matchers.everyItem;
import static org.hamcrest.Matchers.hasItems;
import static org.hamcrest.Matchers.hasSize;
import static org.hamcrest.Matchers.in;
import static org.hamcrest.Matchers.not;
import static org.hamcrest.Matchers.notNullValue;
public class DataStreamTests extends AbstractXContentSerializingTestCase<DataStream> {