Upgrading version in a cluster with closed indices

I've just given the Elasticsearch migration plugin a whirl on an ELK stack, I have lots of closed indices containing historical log data so I get a lot of ancient index warnings.

If I want to upgrade (in my case from 1.7 to 2.x) would it be prudent to reopen all closed indices? Or do I just need to open them and check if they are viable using the migration plugin?

I would reopen them (one by one or rolling) and run the migration plugin again.

That makes sense, I could open up a bunch at once, scan then fix.

You should also disable shard rebalance in cluster while opening such closed indexes (because ES may try automatically rebalance them).

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