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
{
            
            Instant currentTime = Instant.now();
            Instant start1 = currentTime.minus(6, ChronoUnit.HOURS);
            Instant end1 = currentTime.minus(2, ChronoUnit.HOURS);
            
            Instant start2 = currentTime.minus(3, ChronoUnit.HOURS);
            Instant end2 = currentTime.plus(2, ChronoUnit.HOURS);
            String dataStreamName = "logs_my-app_prod";
            var e = expectThrows(
                IllegalArgumentException.class,
                () -> DataStreamTestHelper.getClusterStateWithDataStream(
                    dataStreamName,
                    List.of(Tuple.tuple(start1, end1), Tuple.tuple(start2, end2))
                )
            );
            var formatter = DateFieldMapper.DEFAULT_DATE_TIME_FORMATTER;
            assertThat(
                e.getMessage(),
                equalTo(
                    "backing index ["
                        + DataStream.getDefaultBackingIndexName(dataStreamName, 1, start1.toEpochMilli())
                        + "] with range ["
                        + formatter.format(start1)
                        + " TO "
                        + formatter.format(end1)
                        + "] is overlapping with backing index ["
                        + DataStream.getDefaultBackingIndexName(dataStreamName, 2, start2.toEpochMilli())
                        + "] with range ["
                        + formatter.format(start2)