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 newBackingIndex      the new index that will be part of the {@code DataStream}
     * @return new {@code DataStream} instance with backing indices that contain replacement index instead of the specified
     * existing index.
     */
    public DataStream replaceBackingIndex(Index existingBackingIndex, Index newBackingIndex) {
        List<Index> backingIndices = new ArrayList<>(indices);
        int backingIndexPosition = backingIndices.indexOf(existingBackingIndex);
        if (backingIndexPosition == -1) {
            throw new IllegalArgumentException(
                String.format(Locale.ROOT, "index [%s] is not part of data stream [%s]", existingBackingIndex.getName(), name)
            );
        }
        if (indices.size() == (backingIndexPosition + 1)) {
            throw new IllegalArgumentException(
                String.format(
                    Locale.ROOT,
                    "cannot replace backing index [%s] of data stream [%s] because it is the write index",
                    existingBackingIndex.getName(),
                    name
                )
            );
        }
        backingIndices.set(backingIndexPosition, newBackingIndex);
        return new DataStream(name, backingIndices, generation + 1, metadata, hidden, replicated, system, allowCustomRouting, indexMode);
    }
    /**
     * Adds the specified index as a backing index and returns a new {@code DataStream} instance with the new combination
     * of backing indices.
     *
     * @param index index to add to the data stream