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
+ IndexMetadata.SETTING_INDEX_HIDDEN
                            + "] setting: ["
                            + String.join(",", globalTemplatesThatUseThisComponent)
                            + "]"
                    );
                }
            }
        }
        final Template finalTemplate = new Template(finalSettings, wrappedMappings, template.template().aliases());
        final ComponentTemplate finalComponentTemplate = new ComponentTemplate(finalTemplate, template.version(), template.metadata());
        if (finalComponentTemplate.equals(existing)) {
            return currentState;
        }
        validateTemplate(finalSettings, wrappedMappings, indicesService);
        validate(name, finalComponentTemplate);
        
        if (templatesUsingComponent.size() > 0) {
            ClusterState tempStateWithComponentTemplateAdded = ClusterState.builder(currentState)
                .metadata(Metadata.builder(currentState.metadata()).put(name, finalComponentTemplate))
                .build();
            Exception validationFailure = null;
            for (Map.Entry<String, ComposableIndexTemplate> entry : templatesUsingComponent.entrySet()) {
                final String composableTemplateName = entry.getKey();
                final ComposableIndexTemplate composableTemplate = entry.getValue();
                try {
                    validateIndexTemplateV2(composableTemplateName, composableTemplate, tempStateWithComponentTemplateAdded);
                } catch (Exception e) {
                    if (validationFailure == null) {
                        validationFailure = new IllegalArgumentException(
                            "updating component template ["