diff --git a/scripts/lib/install b/scripts/lib/install
index 3bd27c2640..62a5e505f1 100755
--- a/scripts/lib/install
+++ b/scripts/lib/install
@@ -318,7 +318,7 @@ fi
 # installation process more seamless.
 if [ -z "$NO_DIST_UPGRADE" ]; then
     if [ "$package_system" = apt ]; then
-        apt-get -y dist-upgrade "${APT_OPTIONS[@]}"
+        apt-get -y --allow-downgrades dist-upgrade "${APT_OPTIONS[@]}"
     elif [ "$package_system" = yum ]; then
         # On CentOS, there is no need to do `yum -y upgrade` because `yum -y
         # update` already does the same thing.
@@ -429,7 +429,7 @@ fi
 "$ZULIP_PATH"/scripts/zulip-puppet-apply -f
 
 if [ "$package_system" = apt ]; then
-    apt-get -y upgrade
+    apt-get -y --allow-downgrades upgrade
 elif [ "$package_system" = yum ]; then
     # No action is required because `yum update` already does upgrade.
     :
diff --git a/scripts/lib/upgrade-zulip-stage-2 b/scripts/lib/upgrade-zulip-stage-2
index a41f201bfc..23bb6403d8 100755
--- a/scripts/lib/upgrade-zulip-stage-2
+++ b/scripts/lib/upgrade-zulip-stage-2
@@ -182,7 +182,7 @@ if glob.glob("/usr/share/postgresql/*/extension/tsearch_extras.control"):
 if not args.skip_puppet:
     logging.info("Upgrading system packages...")
     subprocess.check_call(["apt-get", "update"])
-    subprocess.check_call(["apt-get", "-y", "upgrade"])
+    subprocess.check_call(["apt-get", "-y", "--allow-downgrades", "upgrade"])
 
 # To bootstrap zulip-puppet-apply, we need to install the system yaml
 # package; new installs get this, but old installs may not have it.
@@ -350,7 +350,7 @@ if classes != new_classes:
 if not args.skip_puppet:
     logging.info("Applying Puppet changes...")
     subprocess.check_call(["./scripts/zulip-puppet-apply", "--force"])
-    subprocess.check_call(["apt-get", "-y", "upgrade"])
+    subprocess.check_call(["apt-get", "-y", "--allow-downgrades", "upgrade"])
 
 if migrations_needed:
     logging.info("Applying database migrations...")
diff --git a/scripts/setup/apt-repos/zulip/bullseye.list b/scripts/setup/apt-repos/zulip/bullseye.list
index 4c6663d160..8e31cac25f 100644
--- a/scripts/setup/apt-repos/zulip/bullseye.list
+++ b/scripts/setup/apt-repos/zulip/bullseye.list
@@ -1,2 +1,5 @@
 deb http://apt.postgresql.org/pub/repos/apt/ bullseye-pgdg main
 deb-src http://apt.postgresql.org/pub/repos/apt/ bullseye-pgdg main
+
+deb http://apt-archive.postgresql.org/pub/repos/apt/ bullseye-pgdg-archive main
+deb-src http://apt-archive.postgresql.org/pub/repos/apt/ bullseye-pgdg-archive main
diff --git a/scripts/setup/apt-repos/zulip/buster.list b/scripts/setup/apt-repos/zulip/buster.list
index 777c6f33d7..cdbebe559e 100644
--- a/scripts/setup/apt-repos/zulip/buster.list
+++ b/scripts/setup/apt-repos/zulip/buster.list
@@ -1,2 +1,5 @@
 deb http://apt.postgresql.org/pub/repos/apt/ buster-pgdg main
 deb-src http://apt.postgresql.org/pub/repos/apt/ buster-pgdg main
+
+deb http://apt-archive.postgresql.org/pub/repos/apt/ buster-pgdg-archive main
+deb-src http://apt-archive.postgresql.org/pub/repos/apt/ buster-pgdg-archive main
diff --git a/scripts/setup/apt-repos/zulip/focal.list b/scripts/setup/apt-repos/zulip/focal.list
index 8578405cfe..595b2ad521 100644
--- a/scripts/setup/apt-repos/zulip/focal.list
+++ b/scripts/setup/apt-repos/zulip/focal.list
@@ -1,5 +1,8 @@
 deb http://apt.postgresql.org/pub/repos/apt/ focal-pgdg main
 deb-src http://apt.postgresql.org/pub/repos/apt/ focal-pgdg main
 
+deb http://apt-archive.postgresql.org/pub/repos/apt/ focal-pgdg-archive main
+deb-src http://apt-archive.postgresql.org/pub/repos/apt/ focal-pgdg-archive main
+
 deb http://ppa.launchpad.net/groonga/ppa/ubuntu focal main
 deb-src http://ppa.launchpad.net/groonga/ppa/ubuntu focal main
diff --git a/scripts/setup/apt-repos/zulip/zulip.pref b/scripts/setup/apt-repos/zulip/zulip.pref
new file mode 100644
index 0000000000..5a5f47059a
--- /dev/null
+++ b/scripts/setup/apt-repos/zulip/zulip.pref
@@ -0,0 +1,19 @@
+Package: postgresql-10 postgresql-client-10 postgresql-server-dev-10
+Pin: version 10.19-*
+Pin-Priority: 1000
+
+Package: postgresql-11 postgresql-client-11 postgresql-server-dev-11
+Pin: version 11.14-*
+Pin-Priority: 1000
+
+Package: postgresql-12 postgresql-client-12 postgresql-server-dev-12
+Pin: version 12.9-*
+Pin-Priority: 1000
+
+Package: postgresql-13 postgresql-client-13 postgresql-server-dev-13
+Pin: version 13.5-*
+Pin-Priority: 1000
+
+Package: postgresql-14 postgresql-client-14 postgresql-server-dev-14
+Pin: version 14.1-*
+Pin-Priority: 1000