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
assertThat(rolloverResponseBody.get("rolled_over"), is(true));
            var oldIndex = getIndex((String) rolloverResponseBody.get("old_index"));
            var newIndex = getIndex((String) rolloverResponseBody.get("new_index"));
            assertThat(getEndTime(oldIndex), equalTo(getStartTime(newIndex)));
            assertThat(getStartTime(oldIndex).isBefore(getEndTime(oldIndex)), is(true));
            assertThat(getEndTime(newIndex).isAfter(getStartTime(newIndex)), is(true));
        }
    }
    public void testMigrateRegularDataStreamToTsdbDataStream() throws Exception {
        
        var putComposableIndexTemplateRequest = new Request("POST", "/_index_template/1");
        putComposableIndexTemplateRequest.setJsonEntity(NON_TSDB_TEMPLATE);
        assertOK(client().performRequest(putComposableIndexTemplateRequest));
        
        int numRollovers = 4;
        int numDocs = 32;
        var currentTime = Instant.now();
        var currentMinus30Days = currentTime.minus(30, ChronoUnit.DAYS);
        Set<Instant> times = new HashSet<>();
        for (int i = 0; i < numRollovers; i++) {
            for (int j = 0; j < numDocs; j++) {
                var indexRequest = new Request("POST", "/k8s/_doc");
                var time = randomValueOtherThanMany(
                    times::contains,
                    () -> Instant.ofEpochMilli(randomLongBetween(currentMinus30Days.toEpochMilli(), currentTime.toEpochMilli()))
                );
                times.add(time);
                indexRequest.setJsonEntity(DOC.replace("$time", formatInstant(time)));
                var response = client().performRequest(indexRequest);
                assertOK(response);