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
@Override
        public void onFailure(Exception e) {
            listener.onFailure(e);
        }
    }
    @Inject
    public MetadataIndexTemplateService(
        ClusterService clusterService,
        MetadataCreateIndexService metadataCreateIndexService,
        IndicesService indicesService,
        IndexScopedSettings indexScopedSettings,
        NamedXContentRegistry xContentRegistry,
        SystemIndices systemIndices,
        IndexSettingProviders indexSettingProviders
    ) {
        this.clusterService = clusterService;
        this.indicesService = indicesService;
        this.metadataCreateIndexService = metadataCreateIndexService;
        this.indexScopedSettings = indexScopedSettings;
        this.xContentRegistry = xContentRegistry;
        this.systemIndices = systemIndices;
        this.indexSettingProviders = indexSettingProviders.getIndexSettingProviders();
    }
    public void removeTemplates(final RemoveRequest request, final ActionListener<AcknowledgedResponse> listener) {
        clusterService.submitStateUpdateTask("remove-index-template [" + request.name + "]", new TemplateClusterStateUpdateTask(listener) {
            @Override
            public ClusterState execute(ClusterState currentState) {
                Set<String> templateNames = new HashSet<>();
                for (Map.Entry<String, IndexTemplateMetadata> cursor : currentState.metadata().templates().entrySet()) {
                    String templateName = cursor.getKey();