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
if (request.mappings != null) {
            try {
                templateBuilder.putMapping(MapperService.SINGLE_MAPPING_NAME, request.mappings);
            } catch (Exception e) {
                throw new MapperParsingException("Failed to parse mapping: {}", e, request.mappings);
            }
        }
        for (Alias alias : request.aliases) {
            AliasMetadata aliasMetadata = AliasMetadata.builder(alias.name())
                .filter(alias.filter())
                .indexRouting(alias.indexRouting())
                .searchRouting(alias.searchRouting())
                .writeIndex(alias.writeIndex())
                .isHidden(alias.isHidden())
                .build();
            templateBuilder.putAlias(aliasMetadata);
        }
        IndexTemplateMetadata template = templateBuilder.build();
        IndexTemplateMetadata existingTemplate = currentState.metadata().templates().get(request.name);
        if (template.equals(existingTemplate)) {
            
            return currentState;
        }
        Metadata.Builder builder = Metadata.builder(currentState.metadata()).put(template);
        logger.info("adding template [{}] for index patterns {}", request.name, request.indexPatterns);
        return ClusterState.builder(currentState).metadata(builder).build();
    }
    /**
     * Finds index templates whose index pattern matched with the given index name. In the case of
     * hidden indices, a template with a match all pattern or global template will not be returned.