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
* @throws IllegalArgumentException if {@code index} is not a backing index or is the current write index of the data stream
     */
    public DataStream removeBackingIndex(Index index) {
        int backingIndexPosition = indices.indexOf(index);
        if (backingIndexPosition == -1) {
            throw new IllegalArgumentException(
                String.format(Locale.ROOT, "index [%s] is not part of data stream [%s]", index.getName(), name)
            );
        }
        if (indices.size() == (backingIndexPosition + 1)) {
            throw new IllegalArgumentException(
                String.format(
                    Locale.ROOT,
                    "cannot remove backing index [%s] of data stream [%s] because it is the write index",
                    index.getName(),
                    name
                )
            );
        }
        List<Index> backingIndices = new ArrayList<>(indices);
        backingIndices.remove(index);
        assert backingIndices.size() == indices.size() - 1;
        return new DataStream(name, backingIndices, generation + 1, metadata, hidden, replicated, system, allowCustomRouting, indexMode);
    }
    /**
     * Replaces the specified backing index with a new index and returns a new {@code DataStream} instance with
     * the modified backing indices. An {@code IllegalArgumentException} is thrown if the index to be replaced
     * is not a backing index for this data stream or if it is the {@code DataStream}'s write index.
     *
     * @param existingBackingIndex the backing index to be replaced