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/javaRestTest/java/org/elasticsearch/datastreams/TsdbDataStreamRestIT.java
import java.time.temporal.ChronoUnit;
import java.util.HashSet;
import java.util.List;
import java.util.Map;
import java.util.Set;
import static org.elasticsearch.cluster.metadata.DataStreamTestHelper.backingIndexEqualTo;
import static org.hamcrest.Matchers.aMapWithSize;
import static org.hamcrest.Matchers.containsInAnyOrder;
import static org.hamcrest.Matchers.containsString;
import static org.hamcrest.Matchers.empty;
import static org.hamcrest.Matchers.equalTo;
import static org.hamcrest.Matchers.hasSize;
import static org.hamcrest.Matchers.is;
import static org.hamcrest.Matchers.notNullValue;
import static org.hamcrest.Matchers.nullValue;
public class TsdbDataStreamRestIT extends ESRestTestCase {
    private static final String COMPONENT_TEMPLATE = """
        {
            "template": {
                "settings": {}
            }
        }
        """;
    private static final String TEMPLATE = """
        {
            "index_patterns": ["k8s*"],
            "template": {
                "settings":{
                    "index": {
                        "number_of_replicas": 0,