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
new ComposableIndexTemplate.DataStreamTemplate(false, false),
                null
            );
            var e = expectThrows(InvalidIndexTemplateException.class, () -> service.addIndexTemplateV2(state, false, "1", indexTemplate));
            assertThat(e.getMessage(), containsString("[index.mode=time_series] requires a non-empty [index.routing_path]"));
        }
        {
            
            var state = ClusterState.EMPTY_STATE;
            var componentTemplate = new ComponentTemplate(
                new Template(null, new CompressedXContent(generateTsdbMapping()), null),
                null,
                null
            );
            state = service.addComponentTemplate(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,
                new ComposableIndexTemplate.DataStreamTemplate(false, false),
                null
            );
            state = service.addIndexTemplateV2(state, false, "1", indexTemplate);
            assertThat(state.getMetadata().templatesV2().get("1"), equalTo(indexTemplate));
        }
        {