Migrating PostgreSQL from one version to another has become pretty easy. Using pg_upgrade it takes a few seconds to upgrade even a 100GiB database. It becomes interesting when you switch platforms, say from 32bit to 64bit, as we are facing it with our switch from Gentoo to NixOS. Our NixOS-based platform is stable enough for customers to use it. Some larger databases benefit especially from larger RAM sizes.
So the question is: How to migrate from 32bit to 64bit with as little downtime as possible?