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
+ "would cause tsdb data streams "
                    + dataStreamsWithNonTsdbTemplate
                    + " to no longer match a data stream template with a time_series index_mode"
            );
        }
    }
    /**
     * Return a map of v1 template names to their index patterns for v1 templates that would overlap
     * with the given v2 template's index patterns.
     */
    public static Map<String, List<String>> findConflictingV1Templates(
        final ClusterState state,
        final String candidateName,
        final List<String> indexPatterns
    ) {
        Automaton v2automaton = Regex.simpleMatchToAutomaton(indexPatterns.toArray(Strings.EMPTY_ARRAY));
        Map<String, List<String>> overlappingTemplates = new HashMap<>();
        for (Map.Entry<String, IndexTemplateMetadata> cursor : state.metadata().templates().entrySet()) {
            String name = cursor.getKey();
            IndexTemplateMetadata template = cursor.getValue();
            Automaton v1automaton = Regex.simpleMatchToAutomaton(template.patterns().toArray(Strings.EMPTY_ARRAY));
            if (Operations.isEmpty(Operations.intersection(v2automaton, v1automaton)) == false) {
                logger.debug(
                    "composable template {} and legacy template {} would overlap: {} <=> {}",
                    candidateName,
                    name,
                    indexPatterns,
                    template.patterns()
                );
                overlappingTemplates.put(name, template.patterns());