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
final ComponentTemplate template,
        final ActionListener<AcknowledgedResponse> listener
    ) {
        clusterService.submitStateUpdateTask(
            "create-component-template [" + name + "], cause [" + cause + "]",
            new TemplateClusterStateUpdateTask(listener) {
                @Override
                public ClusterState execute(ClusterState currentState) throws Exception {
                    return addComponentTemplate(currentState, create, name, template);
                }
            },
            ClusterStateTaskConfig.build(Priority.URGENT, masterTimeout),
            TEMPLATE_TASK_EXECUTOR
        );
    }
    
    public ClusterState addComponentTemplate(
        final ClusterState currentState,
        final boolean create,
        final String name,
        final ComponentTemplate template
    ) throws Exception {
        final ComponentTemplate existing = currentState.metadata().componentTemplates().get(name);
        if (create && existing != null) {
            throw new IllegalArgumentException("component template [" + name + "] already exists");
        }
        CompressedXContent mappings = template.template().mappings();
        CompressedXContent wrappedMappings = wrapMappingsIfNecessary(mappings, xContentRegistry);
        
        Settings finalSettings = template.template().settings();