The overarching epic is
T133120 "Upgrade Wikimedia search cluster to use Elasticsearch 2.3". There are a lot of subtasks of this epic; take a look at the task description and blocking tasks for more information. This epic blocks
T133119 "Upgrade completion suggester so that its index updates in real time", which is the second part of the goal.
Hopefully, everything should be clear and organised. I've laid all the tasks out in the "This Quarter" of the new
discovery-search-backlog project. Let me know if there are any questions, and I'd be happy to answer them.