Enforcing best_compression on already optimized indices

As per the topic, is that possible, or do I have to re-index?


You can apply it dynacmically - https://www.elastic.co/guide/en/elasticsearch/reference/2.3/index-modules.html#index-codec

I have it applied, but for an index that's already force merged down to 1 segment and has no more indexing activity, is there a way to get that enforced so as to save on disk space without re-indexing?

Yes, apply the setting, that's it.

Is there a way to get that setting to be applicable to currently existing documents?