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
assert candidates.size() > 0 : "we should have returned early with no candidates";
        ComposableIndexTemplate winner = candidates.get(0);
        String winnerName = matchedTemplates.get(winner);
        
        
        
        
        if (winner.indexPatterns().stream().anyMatch(Regex::isMatchAllPattern)
            && IndexMetadata.INDEX_HIDDEN_SETTING.exists(resolveSettings(metadata, winnerName))) {
            throw new IllegalStateException(
                "global index template ["
                    + winnerName
                    + "], composed of component templates ["
                    + String.join(",", winner.composedOf())
                    + "] defined the index.hidden setting, which is not allowed"
            );
        }
        return winnerName;
    }
    /**
     * Collect the given v2 template into an ordered list of mappings.
     */
    public static List<CompressedXContent> collectMappings(final ClusterState state, final String templateName, final String indexName) {
        final ComposableIndexTemplate template = state.metadata().templatesV2().get(templateName);
        assert template != null
            : "attempted to resolve mappings for a template [" + templateName + "] that did not exist in the cluster state";
        if (template == null) {
            return List.of();
        }
        final Map<String, ComponentTemplate> componentTemplates = state.metadata().componentTemplates();