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
throw new IllegalArgumentException(
                "unable to remove composable templates "
                    + new TreeSet<>(templateNames)
                    + " as they are in use by a data streams "
                    + new TreeSet<>(dataStreamsUsingTemplates)
            );
        }
        Metadata.Builder metadata = Metadata.builder(currentState.metadata());
        for (String templateName : templateNames) {
            logger.info("removing index template [{}]", templateName);
            metadata.removeIndexTemplate(templateName);
        }
        return ClusterState.builder(currentState).metadata(metadata).build();
    }
    static Set<String> dataStreamsUsingTemplates(final ClusterState state, final Set<String> templateNames) {
        Metadata metadata = state.metadata();
        Set<String> namePatterns = templateNames.stream()
            .map(templateName -> metadata.templatesV2().get(templateName))
            .filter(Objects::nonNull)
            .map(ComposableIndexTemplate::indexPatterns)
            .map(Set::copyOf)
            .reduce(Sets::union)
            .orElse(Set.of());
        return metadata.dataStreams()
            .values()
            .stream()
            
            .filter(ds -> namePatterns.stream().anyMatch(pattern -> Regex.simpleMatch(pattern, ds.getName())))
            .filter(ds -> {