The stored data format isn't compatible between major versions. Because of that, upgrades are either stressfull and cumbersome (ie. slony + switchover to a pro

The stored data format isn't compatible between major versions. Because of that, upgrades are either stressfull and cumbersome (ie. slony + switchover to a promoted and upgraded slave), or imply a large downtime (ie. pg_upgrade). Also, because of that (WAL format), you can't use native replication between different major versions of PostgreSQL.