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/test/java/org/elasticsearch/cluster/metadata/DataStreamTests.java
}
            {
                
                dataStream.validate(
                    (index) -> IndexMetadata.builder(index)
                        .settings(
                            Settings.builder()
                                .put(IndexMetadata.INDEX_NUMBER_OF_SHARDS_SETTING.getKey(), 1)
                                .put(IndexMetadata.INDEX_NUMBER_OF_REPLICAS_SETTING.getKey(), 1)
                                .put(IndexMetadata.SETTING_INDEX_VERSION_CREATED.getKey(), Version.CURRENT)
                                .build()
                        )
                        .build()
                );
            }
            {
                
                Instant start3 = currentTime.minus(6, ChronoUnit.HOURS);
                Instant end3 = currentTime.plus(2, ChronoUnit.HOURS);
                var e = expectThrows(
                    IllegalArgumentException.class,
                    () -> dataStream.validate(
                        (index) -> IndexMetadata.builder(index)
                            .settings(
                                Settings.builder()
                                    .put(IndexMetadata.INDEX_NUMBER_OF_SHARDS_SETTING.getKey(), 1)
                                    .put(IndexMetadata.INDEX_NUMBER_OF_REPLICAS_SETTING.getKey(), 1)
                                    .put(IndexMetadata.SETTING_INDEX_VERSION_CREATED.getKey(), Version.CURRENT)
                                    .put(IndexSettings.MODE.getKey(), IndexMode.TIME_SERIES)
                                    .put(IndexSettings.TIME_SERIES_START_TIME.getKey(), start3.toEpochMilli())