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
* @param validate should we throw {@link IllegalArgumentException} if conflicts are found or just compute them
     * @return a map of v2 template names to their index patterns for v2 templates that would overlap with the given template
     */
    public Map<String, List<String>> v2TemplateOverlaps(
        ClusterState currentState,
        String name,
        final ComposableIndexTemplate template,
        boolean validate
    ) {
        Map<String, List<String>> overlaps = findConflictingV2Templates(
            currentState,
            name,
            template.indexPatterns(),
            true,
            template.priorityOrZero()
        );
        overlaps.remove(name);
        if (validate && overlaps.size() > 0) {
            String error = String.format(
                Locale.ROOT,
                "index template [%s] has index patterns %s matching patterns from "
                    + "existing templates [%s] with patterns (%s) that have the same priority [%d], multiple index templates may not "
                    + "match during index creation, please use a different priority",
                name,
                template.indexPatterns(),
                Strings.collectionToCommaDelimitedString(overlaps.keySet()),
                overlaps.entrySet().stream().map(e -> e.getKey() + " => " + e.getValue()).collect(Collectors.joining(",")),
                template.priorityOrZero()
            );
            throw new IllegalArgumentException(error);