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 DataStream(
            name,
            reconciledIndices,
            generation,
            metadata == null ? null : new HashMap<>(metadata),
            hidden,
            replicated,
            system,
            allowCustomRouting,
            indexMode
        );
    }
    /**
     * Generates the name of the index that conforms to the default naming convention for backing indices
     * on data streams given the specified data stream name and generation and the current system time.
     *
     * @param dataStreamName name of the data stream
     * @param generation generation of the data stream
     * @return backing index name
     */
    public static String getDefaultBackingIndexName(String dataStreamName, long generation) {
        return getDefaultBackingIndexName(dataStreamName, generation, System.currentTimeMillis());
    }
    /**
     * Generates the name of the index that conforms to the default naming convention for backing indices
     * on data streams given the specified data stream name, generation, and time.
     *
     * @param dataStreamName name of the data stream
     * @param generation generation of the data stream
     * @param epochMillis creation time for the backing index