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
long priority
    ) {
        Automaton v1automaton = Regex.simpleMatchToAutomaton(indexPatterns.toArray(Strings.EMPTY_ARRAY));
        Map<String, List<String>> overlappingTemplates = new TreeMap<>();
        for (Map.Entry<String, ComposableIndexTemplate> entry : state.metadata().templatesV2().entrySet()) {
            String name = entry.getKey();
            ComposableIndexTemplate template = entry.getValue();
            Automaton v2automaton = Regex.simpleMatchToAutomaton(template.indexPatterns().toArray(Strings.EMPTY_ARRAY));
            if (Operations.isEmpty(Operations.intersection(v1automaton, v2automaton)) == false) {
                if (checkPriority == false || priority == template.priorityOrZero()) {
                    logger.debug(
                        "legacy template {} and composable template {} would overlap: {} <=> {}",
                        candidateName,
                        name,
                        indexPatterns,
                        template.indexPatterns()
                    );
                    overlappingTemplates.put(name, template.indexPatterns());
                }
            }
        }
        
        
        overlappingTemplates.remove(candidateName);
        return overlappingTemplates;
    }
    /**
     * Remove the given index template from the cluster state. The index template name
     * supports simple regex wildcards for removing multiple index templates at a time.
     */