[reitanuki]

I'm not familiar this much with MySQL then: if you upgrade your MySQL database, can you still roll back to the old version? If you can't then this is the same restriction when you use the `pg_upgrade` tool in link mode.

(Note that one approach if concerned about the potential for downtime if it goes wrong might be to `pg_upgrade` your secondary in a hot-standby replication setup, then failover and `pg_upgrade` the former primary. In practice, `pg_upgrade` runs very quickly — seconds even on multi-terabyte databases — afaik it doesn't touch the actual data but just the metadata, but read the man pages for more intricate details on that :)).