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
.numberOfShards(1)
                .numberOfReplicas(1)
                .build();
            builder.put(im, false);
        }
        DataStream ds1 = DataStreamTestHelper.newInstance(dsName1, indices1);
        DataStream ds2 = DataStreamTestHelper.newInstance(dsName2, indices2);
        builder.put(ds1);
        builder.put(ds2);
        Index indexToAdd = randomFrom(indices2.toArray(Index.EMPTY_ARRAY));
        IllegalArgumentException e = expectThrows(IllegalArgumentException.class, () -> ds1.addBackingIndex(builder.build(), indexToAdd));
        assertThat(
            e.getMessage(),
            equalTo(
                String.format(
                    Locale.ROOT,
                    "cannot add index [%s] to data stream [%s] because it is already a backing index on data stream [%s]",
                    indexToAdd.getName(),
                    ds1.getName(),
                    ds2.getName()
                )
            )
        );
    }
    public void testAddExistingBackingIndex() {
        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++) {