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
assertOK(client().performRequest(request));
        
        request = new Request("POST", "/_index_template/1");
        request.setJsonEntity(TEMPLATE);
        assertOK(client().performRequest(request));
    }
    public void testTsdbDataStreams() throws Exception {
        var bulkRequest = new Request("POST", "/k8s/_bulk");
        bulkRequest.setJsonEntity(BULK.replace("$now", formatInstant(Instant.now())));
        bulkRequest.addParameter("refresh", "true");
        var response = client().performRequest(bulkRequest);
        assertOK(response);
        var responseBody = entityAsMap(response);
        assertThat("errors in response:\n " + responseBody, responseBody.get("errors"), equalTo(false));
        var getDataStreamsRequest = new Request("GET", "/_data_stream");
        response = client().performRequest(getDataStreamsRequest);
        assertOK(response);
        var dataStreams = entityAsMap(response);
        assertThat(ObjectPath.evaluate(dataStreams, "data_streams"), hasSize(1));
        assertThat(ObjectPath.evaluate(dataStreams, "data_streams.0.name"), equalTo("k8s"));
        assertThat(ObjectPath.evaluate(dataStreams, "data_streams.0.generation"), equalTo(1));
        assertThat(ObjectPath.evaluate(dataStreams, "data_streams.0.template"), equalTo("1"));
        assertThat(ObjectPath.evaluate(dataStreams, "data_streams.0.indices"), hasSize(1));
        String firstBackingIndex = ObjectPath.evaluate(dataStreams, "data_streams.0.indices.0.index_name");
        assertThat(firstBackingIndex, backingIndexEqualTo("k8s", 1));
        var indices = getIndex(firstBackingIndex);
        var escapedBackingIndex = firstBackingIndex.replace(".", "\\.");
        assertThat(ObjectPath.evaluate(indices, escapedBackingIndex + ".data_stream"), equalTo("k8s"));
        assertThat(ObjectPath.evaluate(indices, escapedBackingIndex + ".settings.index.mode"), equalTo("time_series"));
        String startTimeFirstBackingIndex = ObjectPath.evaluate(indices, escapedBackingIndex + ".settings.index.time_series.start_time");