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 (Regex.simpleMatch(names[0], templateName)) {
                    templateNames.add(templateName);
                }
            }
            if (templateNames.isEmpty()) {
                
                
                if (Regex.isMatchAllPattern(names[0])) {
                    return currentState;
                }
                throw new ResourceNotFoundException(names[0]);
            }
        }
        Metadata.Builder metadata = Metadata.builder(currentState.metadata());
        for (String templateName : templateNames) {
            logger.info("removing component template [{}]", templateName);
            metadata.removeComponentTemplate(templateName);
        }
        return ClusterState.builder(currentState).metadata(metadata).build();
    }
    /**
     * Validates that the given component template is not used by any index
     * templates, throwing an error if it is still in use
     */
    static void validateNotInUse(Metadata metadata, String... templateNameOrWildcard) {
        final Predicate<String> predicate;
        if (templateNameOrWildcard.length > 1) {
            predicate = name -> Arrays.asList(templateNameOrWildcard).contains(name);
        } else {
            predicate = name -> Regex.simpleMatch(templateNameOrWildcard[0], name);