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
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);
            assertThat(dataStream, notNullValue());
            assertThat(dataStream.getIndices(), hasSize(2));
            assertThat(
                IndexSettings.TIME_SERIES_START_TIME.get(clusterState.getMetadata().index(dataStream.getIndices().get(0)).getSettings()),
                equalTo(start1)
            );
            assertThat(
                IndexSettings.TIME_SERIES_END_TIME.get(clusterState.getMetadata().index(dataStream.getIndices().get(0)).getSettings()),
                equalTo(end1)
            );
            assertThat(
                IndexSettings.TIME_SERIES_START_TIME.get(clusterState.getMetadata().index(dataStream.getIndices().get(1)).getSettings()),
                equalTo(start2)
            );
            assertThat(
                IndexSettings.TIME_SERIES_END_TIME.get(clusterState.getMetadata().index(dataStream.getIndices().get(1)).getSettings()),
                equalTo(end2)
            );
            
            DataStreamTestHelper.getClusterStateWithDataStream(
                dataStreamName,
                List.of(Tuple.tuple(start1, end1.minus(1, ChronoUnit.MINUTES)), Tuple.tuple(start2.plus(1, ChronoUnit.MINUTES), end2))
            );
        }