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
var responseBody = entityAsMap(response);
                
                assertThat((String) responseBody.get("_index"), backingIndexEqualTo("k8s", i + 1));
            }
            var rolloverRequest = new Request("POST", "/k8s/_rollover");
            var rolloverResponse = client().performRequest(rolloverRequest);
            assertOK(rolloverResponse);
            var rolloverResponseBody = entityAsMap(rolloverResponse);
            assertThat(rolloverResponseBody.get("rolled_over"), is(true));
        }
        var getDataStreamsRequest = new Request("GET", "/_data_stream");
        var getDataStreamResponse = client().performRequest(getDataStreamsRequest);
        assertOK(getDataStreamResponse);
        var dataStreams = entityAsMap(getDataStreamResponse);
        assertThat(ObjectPath.evaluate(dataStreams, "data_streams.0.name"), equalTo("k8s"));
        assertThat(ObjectPath.evaluate(dataStreams, "data_streams.0.generation"), equalTo(5));
        for (int i = 0; i < 5; i++) {
            String backingIndex = ObjectPath.evaluate(dataStreams, "data_streams.0.indices." + i + ".index_name");
            assertThat(backingIndex, backingIndexEqualTo("k8s", i + 1));
            var indices = getIndex(backingIndex);
            var escapedBackingIndex = backingIndex.replace(".", "\\.");
            assertThat(ObjectPath.evaluate(indices, escapedBackingIndex + ".data_stream"), equalTo("k8s"));
            assertThat(ObjectPath.evaluate(indices, escapedBackingIndex + ".settings.index.mode"), nullValue());
            assertThat(ObjectPath.evaluate(indices, escapedBackingIndex + ".settings.index.time_series.start_time"), nullValue());
            assertThat(ObjectPath.evaluate(indices, escapedBackingIndex + ".settings.index.time_series.end_time"), nullValue());
        }
        
        putComposableIndexTemplateRequest = new Request("POST", "/_index_template/1");
        putComposableIndexTemplateRequest.setJsonEntity(TEMPLATE);
        assertOK(client().performRequest(putComposableIndexTemplateRequest));