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
return innerRemoveComponentTemplate(currentState, names);
                }
            },
            ClusterStateTaskConfig.build(Priority.URGENT, masterTimeout),
            TEMPLATE_TASK_EXECUTOR
        );
    }
    
    public static ClusterState innerRemoveComponentTemplate(ClusterState currentState, String... names) {
        validateNotInUse(currentState.metadata(), names);
        final Set<String> templateNames = new HashSet<>();
        if (names.length > 1) {
            Set<String> missingNames = null;
            for (String name : names) {
                if (currentState.metadata().componentTemplates().containsKey(name)) {
                    templateNames.add(name);
                } else {
                    
                    
                    if (missingNames == null) {
                        missingNames = new LinkedHashSet<>();
                    }
                    missingNames.add(name);
                }
            }
            if (missingNames != null) {
                throw new ResourceNotFoundException(String.join(",", missingNames));
            }
        } else {
            for (String templateName : currentState.metadata().componentTemplates().keySet()) {