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 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 newIndex = (String) rolloverResponseBody.get("new_index");
        assertThat(newIndex, backingIndexEqualTo("k8s", 6));
        
        var t = currentTime;
        for (int i = 0; i < numDocs; i++) {
            var indexRequest = new Request("POST", "/k8s/_doc");
            indexRequest.setJsonEntity(DOC.replace("$time", formatInstant(t)));
            var response = client().performRequest(indexRequest);
            assertOK(response);
            var responseBody = entityAsMap(response);
            assertThat((String) responseBody.get("_index"), backingIndexEqualTo("k8s", 6));
            t = t.plusMillis(1000);
        }
        
        var indexRequest = new Request("POST", "/k8s/_doc");
        indexRequest.setJsonEntity(DOC.replace("$time", formatInstant(currentMinus30Days)));
        var e = expectThrows(ResponseException.class, () -> client().performRequest(indexRequest));
        assertThat(e.getMessage(), containsString("is outside of ranges of currently writable indices"));
    }
    public void testChangeTemplateIndexMode() throws Exception {
        {
            var indexRequest = new Request("POST", "/k8s/_doc");
            var time = Instant.now();
            indexRequest.setJsonEntity(DOC.replace("$time", formatInstant(time)));
            var response = client().performRequest(indexRequest);