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
dataStreamName
                )
            )
        );
    }
    public void testAddBackingIndex() {
        int numBackingIndices = randomIntBetween(2, 32);
        String dataStreamName = randomAlphaOfLength(10).toLowerCase(Locale.ROOT);
        final long epochMillis = System.currentTimeMillis();
        List<Index> indices = new ArrayList<>(numBackingIndices);
        for (int k = 1; k <= numBackingIndices; k++) {
            indices.add(new Index(DataStream.getDefaultBackingIndexName(dataStreamName, k, epochMillis), UUIDs.randomBase64UUID(random())));
        }
        Metadata.Builder builder = Metadata.builder();
        for (int k = 1; k <= numBackingIndices; k++) {
            IndexMetadata im = IndexMetadata.builder(indices.get(k - 1).getName())
                .settings(settings(Version.CURRENT))
                .numberOfShards(1)
                .numberOfReplicas(1)
                .build();
            builder.put(im, false);
        }
        DataStream original = DataStreamTestHelper.newInstance(dataStreamName, indices);
        builder.put(original);
        Index indexToAdd = new Index(randomAlphaOfLength(4), UUIDs.randomBase64UUID(random()));
        builder.put(
            IndexMetadata.builder(indexToAdd.getName()).settings(settings(Version.CURRENT)).numberOfShards(1).numberOfReplicas(1).build(),
            false
        );
        DataStream updated = original.addBackingIndex(builder.build(), indexToAdd);