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
String matchedTemplate = findV2Template(metadata, ds.getName(), ds.isHidden());
                
                return templateNames.contains(matchedTemplate);
            })
            .map(DataStream::getName)
            .collect(Collectors.toSet());
    }
    public void putTemplate(final PutRequest request, final ActionListener<AcknowledgedResponse> listener) {
        Settings.Builder updatedSettingsBuilder = Settings.builder();
        updatedSettingsBuilder.put(request.settings).normalizePrefix(IndexMetadata.INDEX_SETTING_PREFIX);
        request.settings(updatedSettingsBuilder.build());
        if (request.name == null) {
            listener.onFailure(new IllegalArgumentException("index_template must provide a name"));
            return;
        }
        if (request.indexPatterns == null) {
            listener.onFailure(new IllegalArgumentException("index_template must provide a template"));
            return;
        }
        try {
            validate(request);
        } catch (Exception e) {
            listener.onFailure(e);
            return;
        }
        final IndexTemplateMetadata.Builder templateBuilder = IndexTemplateMetadata.builder(request.name);
        clusterService.submitStateUpdateTask(
            "create-index-template [" + request.name + "], cause [" + request.cause + "]",
            new TemplateClusterStateUpdateTask(listener) {
                @Override