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
return collectMappings(template, componentTemplates, indexName);
    }
    /**
     * Collect the given v2 template into an ordered list of mappings.
     */
    public static List<CompressedXContent> collectMappings(
        final ComposableIndexTemplate template,
        final Map<String, ComponentTemplate> componentTemplates,
        final String indexName
    ) {
        Objects.requireNonNull(template, "Composable index template must be provided");
        List<CompressedXContent> mappings = template.composedOf()
            .stream()
            .map(componentTemplates::get)
            .filter(Objects::nonNull)
            .map(ComponentTemplate::template)
            .map(Template::mappings)
            .filter(Objects::nonNull)
            .collect(Collectors.toCollection(LinkedList::new));
        
        Optional.ofNullable(template.template()).map(Template::mappings).ifPresent(mappings::add);
        if (template.getDataStreamTemplate() != null && indexName.startsWith(DataStream.BACKING_INDEX_PREFIX)) {
            
            
            if (template.getDataStreamTemplate().isAllowCustomRouting()) {
                mappings.add(0, DEFAULT_TIMESTAMP_MAPPING_WITH_ROUTING);
            } else {
                mappings.add(0, DEFAULT_TIMESTAMP_MAPPING);
            }
        }