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
* Resolve the given v2 template and component templates into an ordered list of aliases
     */
    static List<Map<String, AliasMetadata>> resolveAliases(
        final ComposableIndexTemplate template,
        final Map<String, ComponentTemplate> componentTemplates
    ) {
        Objects.requireNonNull(template, "attempted to resolve aliases for a null template");
        Objects.requireNonNull(componentTemplates, "attempted to resolve aliases with null component templates");
        List<Map<String, AliasMetadata>> aliases = template.composedOf()
            .stream()
            .map(componentTemplates::get)
            .filter(Objects::nonNull)
            .map(ComponentTemplate::template)
            .map(Template::aliases)
            .filter(Objects::nonNull)
            .collect(Collectors.toCollection(ArrayList::new));
        
        Optional.ofNullable(template.template()).map(Template::aliases).ifPresent(aliases::add);
        
        
        
        Collections.reverse(aliases);
        return Collections.unmodifiableList(aliases);
    }
    /**
     * Given a state and a composable template, validate that the final composite template
     * generated by the composable template and all of its component templates contains valid
     * settings, mappings, and aliases.
     */
    private static void validateCompositeTemplate(