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.
server/src/main/java/org/elasticsearch/cluster/metadata/MetadataIndexTemplateService.java
final boolean create,
        final String name,
        final TimeValue masterTimeout,
        final ComposableIndexTemplate template,
        final ActionListener<AcknowledgedResponse> listener
    ) {
        validateV2TemplateRequest(clusterService.state().metadata(), name, template);
        clusterService.submitStateUpdateTask(
            "create-index-template-v2 [" + name + "], cause [" + cause + "]",
            new TemplateClusterStateUpdateTask(listener) {
                @Override
                public ClusterState execute(ClusterState currentState) throws Exception {
                    return addIndexTemplateV2(currentState, create, name, template);
                }
            },
            ClusterStateTaskConfig.build(Priority.URGENT, masterTimeout),
            TEMPLATE_TASK_EXECUTOR
        );
    }
    public static void validateV2TemplateRequest(Metadata metadata, String name, ComposableIndexTemplate template) {
        if (template.indexPatterns().stream().anyMatch(Regex::isMatchAllPattern)) {
            Settings mergedSettings = resolveSettings(template, metadata.componentTemplates());
            if (IndexMetadata.INDEX_HIDDEN_SETTING.exists(mergedSettings)) {
                throw new InvalidIndexTemplateException(
                    name,
                    "global composable templates may not specify the setting " + IndexMetadata.INDEX_HIDDEN_SETTING.getKey()
                );
            }
        }
        final Map<String, ComponentTemplate> componentTemplates = metadata.componentTemplates();