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
if (Regex.simpleMatch(request.name, templateName)) {
                        templateNames.add(templateName);
                    }
                }
                if (templateNames.isEmpty()) {
                    
                    
                    if (Regex.isMatchAllPattern(request.name)) {
                        return currentState;
                    }
                    throw new IndexTemplateMissingException(request.name);
                }
                Metadata.Builder metadata = Metadata.builder(currentState.metadata());
                for (String templateName : templateNames) {
                    logger.info("removing template [{}]", templateName);
                    metadata.removeTemplate(templateName);
                }
                return ClusterState.builder(currentState).metadata(metadata).build();
            }
        }, ClusterStateTaskConfig.build(Priority.URGENT, request.masterTimeout), TEMPLATE_TASK_EXECUTOR);
    }
    /**
     * Add the given component template to the cluster state. If {@code create} is true, an
     * exception will be thrown if the component template already exists
     */
    public void putComponentTemplate(
        final String cause,
        final boolean create,
        final String name,
        final TimeValue masterTimeout,