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
public static final String DEFAULT_TIMESTAMP_FIELD = "@timestamp";
    public static final CompressedXContent DEFAULT_TIMESTAMP_MAPPING;
    private static final CompressedXContent DEFAULT_TIMESTAMP_MAPPING_WITH_ROUTING;
    static {
        final Map<String, Map<String, String>> defaultTimestampField = Map.of(
            DEFAULT_TIMESTAMP_FIELD,
            Map.of("type", DateFieldMapper.CONTENT_TYPE)
        );
        try {
            DEFAULT_TIMESTAMP_MAPPING = new CompressedXContent(
                (builder, params) -> builder.startObject(MapperService.SINGLE_MAPPING_NAME)
                    .field("properties", defaultTimestampField)
                    .endObject()
            );
            DEFAULT_TIMESTAMP_MAPPING_WITH_ROUTING = new CompressedXContent(
                (builder, params) -> builder.startObject(MapperService.SINGLE_MAPPING_NAME)
                    .startObject(RoutingFieldMapper.NAME)
                    .field("required", true)
                    .endObject()
                    .field("properties")
                    .map(defaultTimestampField)
                    .endObject()
            );
        } catch (IOException e) {
            throw new AssertionError(e);
        }
    }
    private static final Logger logger = LogManager.getLogger(MetadataIndexTemplateService.class);
    private final ClusterService clusterService;