[thatwasunusual]

I worked for a company that did a similar "upgrade by replication", but with MySQL. It's quite a few years ago, so I don't remember the versions involved, but it was quite straight-forward once we had done _weeks_ of test runs on a dev environment.

One invaluable thing, though: our application was from the beginning designed to do 100% of all the reads from a read-only slave _if the slave was up to sync_ (which it was 95% of the time). We could also identify testers/developers in the application itself, so we had them using the upgraded slave for two weeks before the actual upgrade.

This made it possible for us to filter out problems in the application/DB-layer, which were few, which means that we probably did a minor version upgrade.

But upgrading by replication is something I can recommend.