> Sadly, PostgreSQL doesn't have a built-in solution for online upgrade yet. This is one of the few points why commercial DBMSes are often worth their money.

I was about to say the same, as much as it pains me to say it. I wonder if Citus or EnterpriseDB has any solutions to a problem like this? Can Citus run temporarily with heterogeneous nodes? E.g. some nodes upgrading with other nodes operating?