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
*
     * @param writeIndex    new write index
     * @param generation    new generation
     * @param timeSeries    whether the template that created this data stream is in time series mode
     *
     * @return new {@code DataStream} instance with the rollover operation applied
     */
    public DataStream rollover(Index writeIndex, long generation, boolean timeSeries) {
        ensureNotReplicated();
        return unsafeRollover(writeIndex, generation, timeSeries);
    }
    /**
     * Like {@link #rollover(Index, long, boolean)}, but does no validation, use with care only.
     */
    public DataStream unsafeRollover(Index writeIndex, long generation, boolean timeSeries) {
        IndexMode indexMode = this.indexMode;
        
        
        if ((indexMode == null || indexMode == IndexMode.STANDARD) && timeSeries) {
            indexMode = IndexMode.TIME_SERIES;
        }
        List<Index> backingIndices = new ArrayList<>(indices);
        backingIndices.add(writeIndex);
        return new DataStream(name, backingIndices, generation, metadata, hidden, false, system, allowCustomRouting, indexMode);
    }
    /**
     * Performs a dummy rollover on a {@code DataStream} instance and returns the tuple of the next write index name and next generation
     * that this {@code DataStream} should roll over to using {@link #rollover(Index, long, boolean)}.
     *
     * @param clusterMetadata Cluster metadata