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
assertThat(reconciledDataStream.isHidden(), equalTo(postSnapshotDataStream.isHidden()));
        assertThat(reconciledDataStream.isReplicated(), equalTo(postSnapshotDataStream.isReplicated()));
        assertThat(reconciledDataStream.getIndices(), everyItem(not(in(indicesToRemove))));
        assertThat(reconciledDataStream.getIndices(), everyItem(not(in(indicesToAdd))));
        assertThat(reconciledDataStream.getIndices().size(), equalTo(preSnapshotDataStream.getIndices().size() - indicesToRemove.size()));
    }
    public void testSnapshotWithAllBackingIndicesRemoved() {
        var preSnapshotDataStream = DataStreamTestHelper.randomInstance();
        var indicesToAdd = new ArrayList<Index>();
        while (indicesToAdd.isEmpty()) {
            
            indicesToAdd.addAll(DataStreamTestHelper.randomIndexInstances());
        }
        var postSnapshotDataStream = new DataStream(
            preSnapshotDataStream.getName(),
            indicesToAdd,
            preSnapshotDataStream.getGeneration(),
            preSnapshotDataStream.getMetadata(),
            preSnapshotDataStream.isHidden(),
            preSnapshotDataStream.isReplicated(),
            preSnapshotDataStream.isSystem(),
            preSnapshotDataStream.isAllowCustomRouting(),
            preSnapshotDataStream.getIndexMode()
        );
        assertNull(postSnapshotDataStream.snapshot(preSnapshotDataStream.getIndices().stream().map(Index::getName).toList()));
    }
    public void testSelectTimeSeriesWriteIndex() {
        Instant currentTime = Instant.now();
        Instant start1 = currentTime.minus(6, ChronoUnit.HOURS);
        Instant end1 = currentTime.minus(2, ChronoUnit.HOURS);