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 (finalSettings != null) {
            finalSettings = Settings.builder().put(finalSettings).normalizePrefix(IndexMetadata.INDEX_SETTING_PREFIX).build();
        }
        
        
        
        final Map<String, ComposableIndexTemplate> templatesUsingComponent = currentState.metadata()
            .templatesV2()
            .entrySet()
            .stream()
            .filter(e -> e.getValue().composedOf().contains(name))
            .collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue));
        
        if (create == false && finalSettings != null) {
            
            if (IndexMetadata.INDEX_HIDDEN_SETTING.exists(finalSettings)) {
                List<String> globalTemplatesThatUseThisComponent = new ArrayList<>();
                for (Map.Entry<String, ComposableIndexTemplate> entry : templatesUsingComponent.entrySet()) {
                    ComposableIndexTemplate templateV2 = entry.getValue();
                    if (templateV2.indexPatterns().stream().anyMatch(Regex::isMatchAllPattern)) {
                        
                        
                        globalTemplatesThatUseThisComponent.add(entry.getKey());
                    }
                }
                if (globalTemplatesThatUseThisComponent.isEmpty() == false) {
                    throw new IllegalArgumentException(
                        "cannot update component template ["
                            + name
                            + "] because the following global templates would resolve to specifying the ["