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
e.getMessage(),
            equalTo(
                String.format(
                    Locale.ROOT,
                    "cannot add index [%s] to data stream [%s] until its alias(es) [%s] are removed",
                    indexToAdd.getName(),
                    original.getName(),
                    Strings.arrayToCommaDelimitedString(aliasNames)
                )
            )
        );
    }
    public void testDefaultBackingIndexName() {
        
        
        long backingIndexNum = randomLongBetween(1, 1000001);
        String dataStreamName = randomAlphaOfLength(6);
        long epochMillis = randomLongBetween(1580536800000L, 1583042400000L);
        String dateString = DataStream.DATE_FORMATTER.formatMillis(epochMillis);
        String defaultBackingIndexName = DataStream.getDefaultBackingIndexName(dataStreamName, backingIndexNum, epochMillis);
        String expectedBackingIndexName = Strings.format(".ds-%s-%s-%06d", dataStreamName, dateString, backingIndexNum);
        assertThat(defaultBackingIndexName, equalTo(expectedBackingIndexName));
    }
    public void testReplaceBackingIndex() {
        int numBackingIndices = randomIntBetween(2, 32);
        int indexToReplace = randomIntBetween(1, numBackingIndices - 1) - 1;
        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())));