For future reference, you can use logical replication to do this online between versions with a minimal window at the end for a final sync when apps are off and not changing data. By that I mean have your 9.6 in prod running as normal, set up your 10/11 newprod and then kick off logical replication for ALL TABLES from prod->newprod. That'll chug along and as long as you have appropriate resources and replication configured correctly, it'll replicate your data over and maintain it while prod is still being used. Then you turn your apps off, let it finish the sync, switch the apps over to the new db and you're good. Obviously there's some missing detail like users, extensions, replication etc, but that's always very specific to an install.

Have you tried pg_upgrade?

Yes. I still don't understand why it failed. It gave me a huge list of locale related reasons it felt unhappy, and since I hadn't selected a locale config option installing either the prior, or the current (9.4 -> 10.x) I felt pretty wierded out.

Postgres 9.4.0 was released 2014-12-18

The version convention has changed with v10.0 so it should be trivial to change from 10.x to 11.x . In your position I'd be updating 9.4.x -> 9.5.x -> 9.6.x -> 10.x