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
+ " TO "
                        + formatter.format(end2)
                        + "]"
                )
            );
        }
        {
            Instant currentTime = Instant.now().truncatedTo(ChronoUnit.MILLIS);
            
            Instant start1 = currentTime.minus(6, ChronoUnit.HOURS);
            Instant end1 = currentTime.minus(2, ChronoUnit.HOURS);
            Instant start2 = currentTime.minus(2, ChronoUnit.HOURS);
            Instant end2 = currentTime.plus(2, ChronoUnit.HOURS);
            String dataStreamName = "logs_my-app_prod";
            var clusterState = DataStreamTestHelper.getClusterStateWithDataStream(
                dataStreamName,
                List.of(Tuple.tuple(start1, end1), Tuple.tuple(start2, end2))
            );
            DataStream dataStream = clusterState.getMetadata().dataStreams().get(dataStreamName);
            {
                
                var e = expectThrows(IllegalStateException.class, () -> dataStream.validate((index) -> null));
                assertThat(
                    e.getMessage(),
                    equalTo(
                        "index ["
                            + DataStream.getDefaultBackingIndexName(dataStreamName, 1, start1.toEpochMilli())
                            + "] is not found in the index metadata supplier"
                    )
                );