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/main/java/org/elasticsearch/cluster/metadata/DataStream.java
return new Tuple<>(im.getIndex().getName(), new Tuple<>(start, end));
                })
                .sorted(Comparator.comparing(entry -> entry.v2().v1())) 
                .toList();
            Tuple<String, Tuple<Instant, Instant>> previous = null;
            var formatter = DateFieldMapper.DEFAULT_DATE_TIME_FORMATTER;
            for (var current : startAndEndTimes) {
                if (previous == null) {
                    previous = current;
                } else {
                    
                    
                    if (previous.v2().v2().compareTo(current.v2().v1()) > 0) {
                        String range1 = formatter.format(previous.v2().v1()) + " TO " + formatter.format(previous.v2().v2());
                        String range2 = formatter.format(current.v2().v1()) + " TO " + formatter.format(current.v2().v2());
                        throw new IllegalArgumentException(
                            "backing index ["
                                + previous.v1()
                                + "] with range ["
                                + range1
                                + "] is overlapping with backing index ["
                                + current.v1()
                                + "] with range ["
                                + range2
                                + "]"
                        );
                    }
                }
            }
        }