They do an in_place upgrade using pg_upgrade I believe. They do have to regenerate system tables - they also automate I think some updates to system statistics.

They do an in_place upgrade using pg_upgrade I believe. They do have to regenerate system tables - they also automate I think some updates to system statistics. But for very large databases this really is the way to go I think. They do suggest a backup before you run this in case there are problems, most folks have periodic backups anyways, but the backup is not used.