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/DataStream.java
public static final String BACKING_INDEX_PREFIX = ".ds-";
    public static final DateFormatter DATE_FORMATTER = DateFormatter.forPattern("uuuu.MM.dd");
    public static final TimestampField TIMESTAMP_FIELD = new DataStream.TimestampField("@timestamp");
    
    public static Comparator<LeafReader> TIMESERIES_LEAF_READERS_SORTER = Comparator.comparingLong((LeafReader r) -> {
        try {
            PointValues points = r.getPointValues(DataStream.TimestampField.FIXED_TIMESTAMP_FIELD);
            if (points != null) {
                byte[] sortValue = points.getMaxPackedValue();
                return LongPoint.decodeDimension(sortValue, 0);
            } else {
                
                
                
                
                return Long.MIN_VALUE;
            }
        } catch (IOException e) {
            throw new ElasticsearchException(
                "Can't access [" + DataStream.TimestampField.FIXED_TIMESTAMP_FIELD + "] field for the index!",
                e
            );
        }
    }).reversed();
    private final LongSupplier timeProvider;
    private final String name;
    private final List<Index> indices;
    private final long generation;