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/test/java/org/elasticsearch/datastreams/MetadataIndexTemplateServiceTests.java
null,
                null,
                100L,
                null,
                null,
                new ComposableIndexTemplate.DataStreamTemplate(false, false),
                null
            );
            service.addIndexTemplateV2(stateWithDS, false, "logs", nonDSTemplate);
        });
        assertThat(
            e.getMessage(),
            containsString(
                "would cause tsdb data streams [logs-mysql-default] to no longer match a data stream template with a time_series index_mode"
            )
        );
    }
    public void testRequireRoutingPath() throws Exception {
        final var service = getMetadataIndexTemplateService();
        {
            
            var componentTemplate = new ComponentTemplate(new Template(null, new CompressedXContent("{}"), null), null, null);
            var state = service.addComponentTemplate(ClusterState.EMPTY_STATE, true, "1", componentTemplate);
            var indexTemplate = new ComposableIndexTemplate(
                Collections.singletonList("logs-*-*"),
                new Template(builder().put("index.mode", "time_series").build(), null, null),
                List.of("1"),
                100L,
                null,
                null,