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
* @return backing index name
     */
    public static String getDefaultBackingIndexName(String dataStreamName, long generation, long epochMillis) {
        return String.format(
            Locale.ROOT,
            BACKING_INDEX_PREFIX + "%s-%s-%06d",
            dataStreamName,
            DATE_FORMATTER.formatMillis(epochMillis),
            generation
        );
    }
    public DataStream(StreamInput in) throws IOException {
        this(
            in.readString(),
            readIndices(in),
            in.readVLong(),
            in.readMap(),
            in.readBoolean(),
            in.readBoolean(),
            in.readBoolean(),
            in.getVersion().onOrAfter(Version.V_8_0_0) ? in.readBoolean() : false,
            in.getVersion().onOrAfter(Version.V_8_1_0) ? in.readOptionalEnum(IndexMode.class) : null
        );
    }
    static List<Index> readIndices(StreamInput in) throws IOException {
        in.readString(); 
        return in.readImmutableList(Index::new);
    }
    public static Diff<DataStream> readDiffFrom(StreamInput in) throws IOException {
        return SimpleDiffable.readDiffFrom(DataStream::new, in);