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
Map<String, List<String>> overlaps = v2TemplateOverlaps(currentState, name, template, validateV2Overlaps);
        overlaps = findConflictingV1Templates(currentState, name, template.indexPatterns());
        if (overlaps.size() > 0) {
            String warning = String.format(
                Locale.ROOT,
                "index template [%s] has index patterns %s matching patterns from "
                    + "existing older templates [%s] with patterns (%s); this template [%s] will take precedence during new index creation",
                name,
                template.indexPatterns(),
                Strings.collectionToCommaDelimitedString(overlaps.keySet()),
                overlaps.entrySet().stream().map(e -> e.getKey() + " => " + e.getValue()).collect(Collectors.joining(",")),
                name
            );
            logger.warn(warning);
            HeaderWarning.addWarning(warning);
        }
        ComposableIndexTemplate finalIndexTemplate = template;
        Template innerTemplate = template.template();
        if (innerTemplate != null) {
            
            Settings finalSettings = innerTemplate.settings();
            if (finalSettings != null) {
                finalSettings = Settings.builder().put(finalSettings).normalizePrefix(IndexMetadata.INDEX_SETTING_PREFIX).build();
            }
            
            
            CompressedXContent mappings = innerTemplate.mappings();
            CompressedXContent wrappedMappings = wrapMappingsIfNecessary(mappings, xContentRegistry);
            final Template finalTemplate = new Template(finalSettings, wrappedMappings, innerTemplate.aliases());
            finalIndexTemplate = new ComposableIndexTemplate(