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 Settings.EMPTY;
        }
        return resolveSettings(template, metadata.componentTemplates());
    }
    /**
     * Resolve the provided v2 template and component templates into a collected {@link Settings} object
     */
    public static Settings resolveSettings(ComposableIndexTemplate template, Map<String, ComponentTemplate> componentTemplates) {
        Objects.requireNonNull(template, "attempted to resolve settings for a null template");
        Objects.requireNonNull(componentTemplates, "attempted to resolve settings with null component templates");
        List<Settings> componentSettings = template.composedOf()
            .stream()
            .map(componentTemplates::get)
            .filter(Objects::nonNull)
            .map(ComponentTemplate::template)
            .map(Template::settings)
            .filter(Objects::nonNull)
            .toList();
        Settings.Builder templateSettings = Settings.builder();
        componentSettings.forEach(templateSettings::put);
        
        Optional.ofNullable(template.template()).map(Template::settings).ifPresent(templateSettings::put);
        return templateSettings.build();
    }
    /**
     * Resolve the given v1 templates into an ordered list of aliases
     */
    public static List<Map<String, AliasMetadata>> resolveAliases(final List<IndexTemplateMetadata> templates) {
        final List<Map<String, AliasMetadata>> resolvedAliases = new ArrayList<>();
        templates.forEach(template -> {