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/main/java/org/elasticsearch/cluster/metadata/DataStream.java
*
     * @return new {@code DataStream} instance with the dummy rollover operation applied
     */
    public Tuple<String, Long> nextWriteIndexAndGeneration(Metadata clusterMetadata) {
        ensureNotReplicated();
        return unsafeNextWriteIndexAndGeneration(clusterMetadata);
    }
    /**
     * Like {@link #nextWriteIndexAndGeneration(Metadata)}, but does no validation, use with care only.
     */
    public Tuple<String, Long> unsafeNextWriteIndexAndGeneration(Metadata clusterMetadata) {
        String newWriteIndexName;
        long generation = this.generation;
        long currentTimeMillis = timeProvider.getAsLong();
        do {
            newWriteIndexName = DataStream.getDefaultBackingIndexName(getName(), ++generation, currentTimeMillis);
        } while (clusterMetadata.hasIndexAbstraction(newWriteIndexName));
        return Tuple.tuple(newWriteIndexName, generation);
    }
    private void ensureNotReplicated() {
        if (replicated) {
            throw new IllegalArgumentException("data stream [" + name + "] cannot be rolled over, because it is a replicated data stream");
        }
    }
    /**
     * Removes the specified backing index and returns a new {@code DataStream} instance with
     * the remaining backing indices.
     *
     * @param index the backing index to remove
     * @return new {@code DataStream} instance with the remaining backing indices