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/test/java/org/elasticsearch/datastreams/DataStreamIndexSettingsProviderTests.java
}
                    }
                }
            }
            """;
        Settings result = provider.getAdditionalIndexSettings(
            DataStream.getDefaultBackingIndexName(dataStreamName, 1),
            dataStreamName,
            true,
            metadata,
            now,
            settings,
            List.of(new CompressedXContent(mapping1), new CompressedXContent(mapping2), new CompressedXContent(mapping3))
        );
        assertThat(result.size(), equalTo(3));
        assertThat(IndexSettings.TIME_SERIES_START_TIME.get(result), equalTo(now.minusMillis(lookAheadTime.getMillis())));
        assertThat(IndexSettings.TIME_SERIES_END_TIME.get(result), equalTo(now.plusMillis(lookAheadTime.getMillis())));
        assertThat(IndexMetadata.INDEX_ROUTING_PATH.get(result), containsInAnyOrder("field1", "field3"));
    }
    public void testGetAdditionalIndexSettingsNoMappings() {
        Metadata metadata = Metadata.EMPTY_METADATA;
        String dataStreamName = "logs-app1";
        Instant now = Instant.now().truncatedTo(ChronoUnit.SECONDS);
        TimeValue lookAheadTime = TimeValue.timeValueHours(2); 
        Settings settings = Settings.EMPTY;
        Settings result = provider.getAdditionalIndexSettings(
            DataStream.getDefaultBackingIndexName(dataStreamName, 1),
            dataStreamName,
            true,
            metadata,