diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 4dbfdb7..561a8fa 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -42,6 +42,33 @@ jobs:
       - name: Test on PostgreSQL ${{ matrix.pg }}, across every TEST_SCHEMA value
         run: make test-schema-all
 
+  # Proves the in-place extension update path: CREATE EXTENSION at the
+  # oldest version we still ship a full install script for (0.9.6), then
+  # ALTER EXTENSION UPDATE (no pg_upgrade, same PostgreSQL), all within
+  # test/install/load.sql's own committed session (TEST_LOAD_SOURCE=update -
+  # see the Makefile) - then run the FULL suite against the updated
+  # database via the SAME expected output as a fresh install (see
+  # test/README.md for how the suite stays schema/load-mode invariant).
+  # No external script needed for this leg: unlike a real pg_upgrade
+  # (pg-upgrade-test), the update itself is just SQL, so test/install can
+  # do the whole thing inside one pg_regress invocation.
+  extension-update-test:
+    strategy:
+      matrix:
+        pg: [18, 17, 16, 15, 14, 13, 12, 11, 10]
+    name: ⬆️ Extension update test on PostgreSQL ${{ matrix.pg }}
+    runs-on: ubuntu-latest
+    container: pgxn/pgxn-tools
+    steps:
+      - name: Start PostgreSQL ${{ matrix.pg }}
+        run: pg-start ${{ matrix.pg }}
+      - name: Check out the repo
+        uses: actions/checkout@v4
+      - name: Install count_nulls
+        run: make install
+      - name: Update 0.9.6 -> current and run the suite
+        run: make verify-results TEST_LOAD_SOURCE=update
+
   pg-tle-test:
     strategy:
       matrix:
diff --git a/Makefile b/Makefile
index 89e001d..1b10f15 100644
--- a/Makefile
+++ b/Makefile
@@ -32,8 +32,8 @@ TEST_SCHEMA ?=
 export PGOPTIONS := $(PGOPTIONS) -c count_nulls.test_schema=$(TEST_SCHEMA)
 
 # Every TEST_SCHEMA value the suite is tested against. A single source so
-# test-schema-all and CI (once collapsed - see the "why not a CI matrix"
-# note below) can't silently drift onto different sets.
+# test-schema-all/test-update-schema-all and CI can't silently drift onto
+# different sets.
 TEST_SCHEMA_VALUES = "" Quoted
 
 # TEST_SCHEMA is deliberately NOT a CI matrix dimension: unlike PostgreSQL
@@ -57,3 +57,35 @@ test-schema-all:
 		echo "=== TEST_SCHEMA=$$schema ==="; \
 		$(MAKE) test TEST_SCHEMA="$$schema" || exit 1; \
 	done
+
+# TEST_LOAD_SOURCE selects how test/install/load.sql installs count_nulls
+# for the WHOLE test run:
+#   - fresh (default): CREATE EXTENSION count_nulls (current version).
+#   - update: CREATE EXTENSION at the oldest version we still ship a full
+#     install script for (0.9.6), then ALTER EXTENSION UPDATE to current -
+#     committed, since test/install runs outside any per-test rolled-back
+#     transaction (see pgxntool/README.asc's Update & Upgrade (U&U) Testing
+#     section for why the commit matters).
+#   - existing: count_nulls is already installed (a real `pg_upgrade` run,
+#     external to this invocation) - test/install only asserts it's present
+#     and current, it does not drop/create/update anything. Meant to be run
+#     with CONTRIB_TESTDB=<db> EXTRA_REGRESS_OPTS=--use-existing against a
+#     real database, not via a make wrapper here.
+#
+# "update" (this) is extension-level (ALTER EXTENSION UPDATE); "upgrade" is
+# cluster-level (pg_upgrade) - 'existing' is how that axis is exercised.
+#
+# Propagated the same way as TEST_SCHEMA: via the count_nulls.test_load_mode
+# GUC, exported unconditionally through PGOPTIONS, read without missing_ok.
+TEST_LOAD_SOURCE ?= fresh
+ifeq ($(filter $(TEST_LOAD_SOURCE),fresh update existing),)
+$(error TEST_LOAD_SOURCE must be 'fresh', 'update' or 'existing', got '$(TEST_LOAD_SOURCE)')
+endif
+export PGOPTIONS := $(PGOPTIONS) -c count_nulls.test_load_mode=$(TEST_LOAD_SOURCE)
+
+# Convenience wrapper: `make test-update` == `make test TEST_LOAD_SOURCE=update`.
+# Must recurse (a fresh $(MAKE)) rather than depend on `test`, so the
+# parse-time TEST_LOAD_SOURCE conditional above re-evaluates with update set.
+.PHONY: test-update
+test-update:
+	$(MAKE) test TEST_LOAD_SOURCE=update
diff --git a/test/install/load.sql b/test/install/load.sql
index 031eafc..5d6d1bf 100644
--- a/test/install/load.sql
+++ b/test/install/load.sql
@@ -34,4 +34,65 @@ CREATE SCHEMA IF NOT EXISTS :"schema";
 SET search_path = :"schema";
 \endif
 
+/*
+ * Mode selection: 'fresh' installs the current version directly; 'update'
+ * installs the oldest version we still ship a full script for (0.9.6) and
+ * runs ALTER EXTENSION UPDATE, committed (this file runs outside any
+ * per-test rolled-back transaction, unlike the old test/deps.sql approach -
+ * see pgxntool/README.asc's U&U section for why the commit matters);
+ * 'existing' asserts count_nulls is already installed (a real `pg_upgrade`
+ * run, external to this invocation) and touches nothing.
+ *
+ * Read without missing_ok, same reasoning as count_nulls.test_schema above.
+ */
+SELECT current_setting('count_nulls.test_load_mode') AS count_nulls_test_load_mode
+\gset
+
+DO $$
+BEGIN
+  IF current_setting('count_nulls.test_load_mode') NOT IN ('fresh', 'update', 'existing') THEN
+    RAISE EXCEPTION
+      'count_nulls.test_load_mode must be ''fresh'', ''update'' or ''existing'', got ''%'''
+      , current_setting('count_nulls.test_load_mode')
+    ;
+  END IF;
+END
+$$;
+
+SELECT :'count_nulls_test_load_mode' = 'update'   AS count_nulls_update_mode
+\gset
+SELECT :'count_nulls_test_load_mode' = 'existing' AS count_nulls_existing_mode
+\gset
+
+\if :count_nulls_existing_mode
+/*
+ * Already installed by something external to this pg_regress invocation
+ * (a real pg_upgrade run - see the pg-upgrade-test CI job). Only assert
+ * it's present and at the current version; do NOT drop/create/update it -
+ * the whole point of this mode is testing the REAL migrated objects.
+ */
+DO $$
+DECLARE
+  v_installed text := (SELECT extversion FROM pg_extension WHERE extname = 'count_nulls');
+  v_default   text := (SELECT default_version FROM pg_available_extensions WHERE name = 'count_nulls');
+BEGIN
+  IF v_installed IS NULL THEN
+    RAISE EXCEPTION 'count_nulls.test_load_mode=existing but count_nulls is not installed';
+  END IF;
+  IF v_installed IS DISTINCT FROM v_default THEN
+    RAISE EXCEPTION 'count_nulls installed at % but default_version is %', v_installed, v_default;
+  END IF;
+END
+$$;
+\elif :count_nulls_update_mode
+CREATE EXTENSION count_nulls VERSION '0.9.6';
+/*
+ * Suppress the "already installed, no update" NOTICE class of messages any
+ * update script might emit.
+ */
+SET client_min_messages = WARNING;
+ALTER EXTENSION count_nulls UPDATE;
+SET client_min_messages = NOTICE;
+\else
 CREATE EXTENSION count_nulls;
+\endif