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 = randomAlphaOfLength(10).toLowerCase(Locale.ROOT);
        List<Index> indices = new ArrayList<>(numBackingIndices);
        for (int i = 1; i <= numBackingIndices; i++) {
            indices.add(new Index(DataStream.getDefaultBackingIndexName(dataStreamName, i), UUIDs.randomBase64UUID(random())));
        }
        int generation = randomBoolean() ? numBackingIndices : numBackingIndices + randomIntBetween(1, 5);
        DataStream original = newInstance(dataStreamName, indices, generation, null);
        Index newBackingIndex = new Index("replacement-index", UUIDs.randomBase64UUID(random()));
        IllegalArgumentException e = expectThrows(
            IllegalArgumentException.class,
            () -> original.replaceBackingIndex(indices.get(writeIndexPosition), newBackingIndex)
        );
        assertThat(
            e.getMessage(),
            equalTo(
                String.format(
                    Locale.ROOT,
                    "cannot replace backing index [%s] of data stream [%s] because it is the write index",
                    indices.get(writeIndexPosition).getName(),
                    dataStreamName
                )
            )
        );
    }
    public void testSnapshot() {
        var preSnapshotDataStream = DataStreamTestHelper.randomInstance();
        var indicesToRemove = randomSubsetOf(preSnapshotDataStream.getIndices());
        if (indicesToRemove.size() == preSnapshotDataStream.getIndices().size()) {
            
            indicesToRemove.remove(0);