Let's assume we had PostgreSQL version X.Y and decided to give a test-try to version Z.W. Probably, there are some changes between X.Y and Z.W that could cause a little bit of trouble :-)
It would be preferable to run Z.W in parallel with X.Y, checking whether it goes well and using only X.Y's replies for a while. Later on, having Z.W's DB "ok" we could use it as primary, discarding X.Y optionally.
Question: Is there any PostgreSQL proxy that can run queries on several servers at once?