I suspect you were thinking of changing the number of shards, which can be an important part of rebalancing an index. Unfortunately, Elasticsearch can't do that.
There is a convenient reindex API available in more recent versions, although in most cases where you can rebuild your index anyway, it's faster to do that.
Ahh. I was wondering when they were going to get around to implementing that. I might have to upgrade to 5.x soon.
FYI, the reindex API was added as experimental in 2.3, so it's been there for about a year [1]. Even as experimental it worked great.
Ha. I just came here to edit my comment. I remember seeing it before and just checked the docs again. In 5.2 it's STILL considered experiential. Craziness. It took me about an hour to build The basic functionality myself.
Thank you for noticing this, it is not considered experimental anymore. I integrated a change[1] to update the docs; it should percolate through to the site within thirty minutes.