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
}
        final Set<String> matchingComponentTemplates = metadata.componentTemplates()
            .keySet()
            .stream()
            .filter(predicate)
            .collect(Collectors.toSet());
        final Set<String> componentsBeingUsed = new HashSet<>();
        final List<String> templatesStillUsing = metadata.templatesV2().entrySet().stream().filter(e -> {
            Set<String> intersecting = Sets.intersection(new HashSet<>(e.getValue().composedOf()), matchingComponentTemplates);
            if (intersecting.size() > 0) {
                componentsBeingUsed.addAll(intersecting);
                return true;
            }
            return false;
        }).map(Map.Entry::getKey).toList();
        if (templatesStillUsing.size() > 0) {
            throw new IllegalArgumentException(
                "component templates "
                    + componentsBeingUsed
                    + " cannot be removed as they are still in use by index templates "
                    + templatesStillUsing
            );
        }
    }
    /**
     * Add the given index template to the cluster state. If {@code create} is true, an
     * exception will be thrown if the component template already exists
     */
    public void putIndexTemplateV2(
        final String cause,