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.
modules/data-streams/src/main/java/org/elasticsearch/datastreams/DataStreamIndexSettingsProvider.java
static final DateFormatter FORMATTER = DateFieldMapper.DEFAULT_DATE_TIME_FORMATTER;
    private final CheckedFunction<IndexMetadata, MapperService, IOException> mapperServiceFactory;
    DataStreamIndexSettingsProvider(CheckedFunction<IndexMetadata, MapperService, IOException> mapperServiceFactory) {
        this.mapperServiceFactory = mapperServiceFactory;
    }
    @Override
    public Settings getAdditionalIndexSettings(
        String indexName,
        String dataStreamName,
        boolean timeSeries,
        Metadata metadata,
        Instant resolvedAt,
        Settings allSettings,
        List<CompressedXContent> combinedTemplateMappings
    ) {
        if (dataStreamName != null) {
            DataStream dataStream = metadata.dataStreams().get(dataStreamName);
            
            
            
            boolean migrating = dataStream != null
                && (dataStream.getIndexMode() == null || dataStream.getIndexMode() == IndexMode.STANDARD)
                && timeSeries;
            IndexMode indexMode;
            if (migrating) {
                indexMode = IndexMode.TIME_SERIES;