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
}
    @Nullable
    public Map<String, Object> getMetadata() {
        return metadata;
    }
    public boolean isHidden() {
        return hidden;
    }
    /**
     * Determines whether this data stream is replicated from elsewhere,
     * for example a remote cluster.
     *
     * @return Whether this data stream is replicated.
     */
    public boolean isReplicated() {
        return replicated;
    }
    public boolean isSystem() {
        return system;
    }
    public boolean isAllowCustomRouting() {
        return allowCustomRouting;
    }
    @Nullable
    public IndexMode getIndexMode() {
        return indexMode;
    }
    /**
     * Performs a rollover on a {@code DataStream} instance and returns a new instance containing
     * the updated list of backing indices and incremented generation.