Elasticsearch 2.0 is now released, but when using the migration plugin to check if we can upgrade smoothly there is an index mapping conflict (more info about the mapping conflicts here) for the phabricator index.
The problem seems to be that the .when field is sometimes pushed as a number field containing a unix timestamp, and sometimes a string field containing a unix timestamp. So with the new strict mapping regime in ES 2.0, this will not be allowed.
Have you been seeing this yourself?
Here is the output from the migration plugin: