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
.put(IndexSettings.TIME_SERIES_END_TIME.getKey(), end3.toEpochMilli())
                                    .build()
                            )
                            .build()
                    )
                );
                var formatter = DateFieldMapper.DEFAULT_DATE_TIME_FORMATTER;
                assertThat(
                    e.getMessage(),
                    equalTo(
                        "backing index ["
                            + DataStream.getDefaultBackingIndexName(dataStreamName, 1, start1.toEpochMilli())
                            + "] with range ["
                            + formatter.format(start3)
                            + " TO "
                            + formatter.format(end3)
                            + "] is overlapping with backing index ["
                            + DataStream.getDefaultBackingIndexName(dataStreamName, 2, start2.toEpochMilli())
                            + "] with range ["
                            + formatter.format(start3)
                            + " TO "
                            + formatter.format(end3)
                            + "]"
                    )
                );
            }
        }
    }
}