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
private final IndicesService indicesService;
    private final MetadataCreateIndexService metadataCreateIndexService;
    private final IndexScopedSettings indexScopedSettings;
    private final NamedXContentRegistry xContentRegistry;
    private final SystemIndices systemIndices;
    private final Set<IndexSettingProvider> indexSettingProviders;
    /**
     * This is the cluster state task executor for all template-based actions.
     */
    private static final SimpleBatchedExecutor<TemplateClusterStateUpdateTask, Void> TEMPLATE_TASK_EXECUTOR =
        new SimpleBatchedExecutor<>() {
            @Override
            public Tuple<ClusterState, Void> executeTask(TemplateClusterStateUpdateTask task, ClusterState clusterState) throws Exception {
                return Tuple.tuple(task.execute(clusterState), null);
            }
            @Override
            public void taskSucceeded(TemplateClusterStateUpdateTask task, Void unused) {
                task.listener.onResponse(AcknowledgedResponse.TRUE);
            }
        };
    /**
     * A specialized cluster state update task that always takes a listener handling an
     * AcknowledgedResponse, as all template actions have simple acknowledged yes/no responses.
     */
    private abstract static class TemplateClusterStateUpdateTask implements ClusterStateTaskListener {
        final ActionListener<AcknowledgedResponse> listener;
        TemplateClusterStateUpdateTask(ActionListener<AcknowledgedResponse> listener) {
            this.listener = listener;
        }
        public abstract ClusterState execute(ClusterState currentState) throws Exception;