In our case at Meltwater we also have new documents as well as updates to old documents coming in. Meaning older indexes are actively being modified with low latency requirements on visibility and consistency of those modifications. This makes it more tricky to both reindex as well as doing a seamless/no-downtime-whatsoever upgrade to a new major version. It's not infeasible at all though, and we're working on it. Reindexing a PB of data should be possible on a matter of weeks based on our estimates, but we shall see!