You should upgrade your CanIt cluster by:
1) Stop CanIt on both machines:
2) Upgrade CanIt on the primary database:
Note that the "Upgrading Database" part may appear to "hang" for a bit because it doesn't print anything; it's not hung, it's just working without printing anything, so do NOT interrupt it.
3) Upgrade CanIt on antispam2 AFTER antispam1 finishes:
Then, if you use Database Failover, you'll need to:
4) Delete the existing WAL files in /var/backups/postgres/wal/:
5) Re-initialise the "base backup":
Because of the link speed between the machines, this could take a long time - let it run to completion!