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
int numConflictingIndices = randomIntBetween(1, 10);
        Metadata.Builder builder = Metadata.builder();
        for (int k = 1; k <= numConflictingIndices; k++) {
            IndexMetadata im = IndexMetadata.builder(DataStream.getDefaultBackingIndexName(ds.getName(), ds.getGeneration() + k, 0L))
                .settings(settings(Version.CURRENT))
                .numberOfShards(1)
                .numberOfReplicas(1)
                .build();
            builder.put(im, false);
        }
        final Tuple<String, Long> newCoordinates = ds.nextWriteIndexAndGeneration(builder.build());
        final DataStream rolledDs = ds.rollover(new Index(newCoordinates.v1(), UUIDs.randomBase64UUID()), newCoordinates.v2(), false);
        assertThat(rolledDs.getName(), equalTo(ds.getName()));
        assertThat(rolledDs.getTimeStampField(), equalTo(ds.getTimeStampField()));
        assertThat(rolledDs.getGeneration(), equalTo(ds.getGeneration() + numConflictingIndices + 1));
        assertThat(rolledDs.getIndices().size(), equalTo(ds.getIndices().size() + 1));
        assertTrue(rolledDs.getIndices().containsAll(ds.getIndices()));
        assertTrue(rolledDs.getIndices().contains(rolledDs.getWriteIndex()));
        assertThat(rolledDs.getIndexMode(), equalTo(ds.getIndexMode()));
    }
    public void testRolloverIndexMode() {
        IndexMode indexMode = randomBoolean() ? IndexMode.STANDARD : null;
        DataStream ds = DataStreamTestHelper.randomInstance().promoteDataStream();
        
        ds = new DataStream(
            ds.getName(),
            ds.getIndices(),
            ds.getGeneration(),
            ds.getMetadata(),
            ds.isHidden(),