Just curious, is the issue a time thing, or is it on a system that can't easily upgrade it - or is it a breaking changes thing that would require refactors?
One main challenge is to upgrade a live large database smoothly, without downtime.
I believe minor version upgrades are easily enough since they maintain disk format compatibility and you can simply spin up a new replica server under the new postgres version and then when it's synched with the main database you can make the switch and kill the old version. I'm not so sure about migrating to a new major version (ex: 9.x to 15.x).
Thing is, in this specific context, as I understand it, this is much easier to do post 9.x.
Notably, Postgres has more formal support for logical replication. Logical replication is distinct from the normal replication in that it forwards actual SQL to the replica rather than binary write ahead logs.
The SQL is portable across versions more than WALs are. So with logical replication you can copy a live db to a new version. Then you just need some (ideally) brief down time to swap out the servers.
There are solutions for 9.x, but it got much better later on.
Basically I'm just curious what broke for you