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
            );
            var state = service.addIndexTemplateV2(ClusterState.EMPTY_STATE, false, "1", indexTemplate);
            assertThat(state.getMetadata().templatesV2().get("1"), equalTo(indexTemplate));
        }
        {
            
            var indexTemplate = new ComposableIndexTemplate(
                Collections.singletonList("logs-*-*"),
                new Template(builder().put("index.mode", "time_series").build(), new CompressedXContent(generateTsdbMapping()), null),
                List.of("1"),
                100L,
                null,
                null,
                new ComposableIndexTemplate.DataStreamTemplate(false, false),
                null
            );
            var state = service.addIndexTemplateV2(ClusterState.EMPTY_STATE, false, "1", indexTemplate);
            assertThat(state.getMetadata().templatesV2().get("1"), equalTo(indexTemplate));
        }
    }
    private MetadataIndexTemplateService getMetadataIndexTemplateService() {
        var indicesService = getInstanceFromNode(IndicesService.class);
        var clusterService = getInstanceFromNode(ClusterService.class);
        var indexSettingProviders = new IndexSettingProviders(Set.of(createSettingsProvider(xContentRegistry())));
        var createIndexService = new MetadataCreateIndexService(
            Settings.EMPTY,
            clusterService,
            indicesService,
            null,