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/main/java/org/elasticsearch/cluster/metadata/MetadataIndexTemplateService.java
import org.elasticsearch.indices.SystemIndices;
import org.elasticsearch.xcontent.NamedXContentRegistry;
import java.io.IOException;
import java.time.Instant;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collections;
import java.util.Comparator;
import java.util.HashMap;
import java.util.HashSet;
import java.util.LinkedHashSet;
import java.util.LinkedList;
import java.util.List;
import java.util.Locale;
import java.util.Map;
import java.util.Objects;
import java.util.Optional;
import java.util.Set;
import java.util.TreeMap;
import java.util.TreeSet;
import java.util.function.Function;
import java.util.function.Predicate;
import java.util.stream.Collectors;
import static java.util.Collections.emptyMap;
import static org.elasticsearch.cluster.metadata.MetadataCreateDataStreamService.validateTimestampFieldMapping;
import static org.elasticsearch.indices.cluster.IndicesClusterStateService.AllocatedIndices.IndexRemovalReason.NO_LONGER_ASSIGNED;
/**
 * Service responsible for submitting index templates updates
 */
public class MetadataIndexTemplateService {