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
for (int k = 1; k <= numBackingIndices; k++) {
            indices.add(new Index(DataStream.getDefaultBackingIndexName(dataStreamName, k), UUIDs.randomBase64UUID(random())));
        }
        DataStream original = DataStreamTestHelper.newInstance(dataStreamName, indices);
        final Index indexToRemove = new Index(randomAlphaOfLength(4), UUIDs.randomBase64UUID(random()));
        IllegalArgumentException e = expectThrows(IllegalArgumentException.class, () -> original.removeBackingIndex(indexToRemove));
        assertThat(
            e.getMessage(),
            equalTo(Strings.format("index [%s] is not part of data stream [%s]", indexToRemove.getName(), dataStreamName))
        );
    }
    public void testRemoveBackingWriteIndex() {
        int numBackingIndices = randomIntBetween(2, 32);
        String dataStreamName = randomAlphaOfLength(10).toLowerCase(Locale.ROOT);
        List<Index> indices = new ArrayList<>(numBackingIndices);
        for (int k = 1; k <= numBackingIndices; k++) {
            indices.add(new Index(DataStream.getDefaultBackingIndexName(dataStreamName, k), UUIDs.randomBase64UUID(random())));
        }
        DataStream original = DataStreamTestHelper.newInstance(dataStreamName, indices);
        IllegalArgumentException e = expectThrows(
            IllegalArgumentException.class,
            () -> original.removeBackingIndex(indices.get(numBackingIndices - 1))
        );
        assertThat(
            e.getMessage(),
            equalTo(
                String.format(
                    Locale.ROOT,
                    "cannot remove backing index [%s] of data stream [%s] because it is the write index",
                    indices.get(numBackingIndices - 1).getName(),