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
}
                        },
                        "another_field": {
                            "type": "keyword"
                        }
                    }
                }
            }
            """;
        Settings result = generateTsdbSettings(mapping, now);
        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("host.id", "prometheus.labels.*"));
    }
    private Settings generateTsdbSettings(String mapping, Instant now) throws IOException {
        Metadata metadata = Metadata.EMPTY_METADATA;
        String dataStreamName = "logs-app1";
        Settings settings = Settings.EMPTY;
        return provider.getAdditionalIndexSettings(
            DataStream.getDefaultBackingIndexName(dataStreamName, 1),
            dataStreamName,
            true,
            metadata,
            now,
            settings,
            List.of(new CompressedXContent(mapping))
        );
    }
}