We see that it's possible with reindexing data with moving from 1.7 to 2.0 instance. But it sees not very efficient. How to upgrade to 2.0 efficiently if we already have indexes that stores _id and _version?
You need to reindex unfortunately.
So sad...
2.0 full of breaking changes, so migration on it is not simple for projects that uses ES for at least year...
While it's not good we see that we should stay on 1.7 for some meaning time...