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
}
        DataStream original = DataStreamTestHelper.newInstance(dataStreamName, indices);
        Index newBackingIndex = new Index("replacement-index", UUIDs.randomBase64UUID(random()));
        DataStream updated = original.replaceBackingIndex(indices.get(indexToReplace), newBackingIndex);
        assertThat(updated.getName(), equalTo(original.getName()));
        assertThat(updated.getGeneration(), equalTo(original.getGeneration() + 1));
        assertThat(updated.getTimeStampField(), equalTo(original.getTimeStampField()));
        assertThat(updated.getIndices().size(), equalTo(numBackingIndices));
        assertThat(updated.getIndices().get(indexToReplace), equalTo(newBackingIndex));
        for (int i = 0; i < numBackingIndices; i++) {
            if (i != indexToReplace) {
                assertThat(updated.getIndices().get(i), equalTo(original.getIndices().get(i)));
            }
        }
    }
    public void testReplaceBackingIndexThrowsExceptionIfIndexNotPartOfDataStream() {
        int numBackingIndices = randomIntBetween(2, 32);
        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())));
        }
        DataStream original = DataStreamTestHelper.newInstance(dataStreamName, indices);
        Index standaloneIndex = new Index("index-foo", UUIDs.randomBase64UUID(random()));
        Index newBackingIndex = new Index("replacement-index", UUIDs.randomBase64UUID(random()));
        expectThrows(IllegalArgumentException.class, () -> original.replaceBackingIndex(standaloneIndex, newBackingIndex));
    }
    public void testReplaceBackingIndexThrowsExceptionIfReplacingWriteIndex() {
        int numBackingIndices = randomIntBetween(2, 32);
        int writeIndexPosition = numBackingIndices - 1;