diff --git a/site-cookbooks/kosmos-postgresql/recipes/default.rb b/site-cookbooks/kosmos-postgresql/recipes/default.rb
index be288d6..2b8e09b 100644
--- a/site-cookbooks/kosmos-postgresql/recipes/default.rb
+++ b/site-cookbooks/kosmos-postgresql/recipes/default.rb
@@ -37,6 +37,19 @@ service postgresql_service do
   action [:enable]
 end
 
+# This will only be run once, if the /var/lib/postgresql/10/main directory
+# exists. The old data directory is then moved.
+execute "upgrade postgresql to 12" do
+  command <<-EOF
+systemctl stop postgresql@12-main
+systemctl stop postgresql@10-main
+su - postgres -c "/usr/lib/postgresql/12/bin/pg_upgrade --old-bindir=/usr/lib/postgresql/10/bin/ --new-bindir=/usr/lib/postgresql/12/bin/ --old-datadir=/etc/postgresql/10/main/ --new-datadir=/etc/postgresql/12/main/"
+mv /var/lib/postgresql/10/main /var/lib/postgresql/10/main.old
+systemctl start postgresql@12-main
+  EOF
+  only_if { ::File.exist? "/var/lib/postgresql/10/main" }
+end
+
 postgresql_replicas.each do |replica|
   postgresql_access "#{replica[:hostname]} replication" do
     access_type "host"