diff --git a/Makefile b/Makefile
index e57011a..98b9c3f 100644
--- a/Makefile
+++ b/Makefile
@@ -2,3 +2,41 @@ include pgxntool/base.mk
 
 # Temporary hack
 testdeps: $(wildcard test/*/*.sql) $(wildcard test/*.sql) # Be careful not to include directories in this
+
+# Install the oldest historical version's full install script so the update
+# test in test/build/upgrade.sql and test/deps.sql's update mode can
+# CREATE EXTENSION count_nulls VERSION '0.9.6'.
+# Not covered by base.mk's DATA wildcard, which only picks up upgrade scripts
+# (sql/*--*--*.sql) and the current version file.
+DATA += sql/count_nulls--0.9.6.sql
+
+# TEST_LOAD_SOURCE selects how test/deps.sql installs the extension for the
+# WHOLE test suite:
+#   - fresh (default): CREATE EXTENSION count_nulls (current version).
+#   - update: CREATE EXTENSION at the oldest version we still ship a full
+#     install script for (0.9.6), then ALTER EXTENSION UPDATE to current.
+# Running the SAME suite with the SAME expected output against the updated
+# database proves it behaves identically to a fresh install.
+#
+# "update" (this) is extension-level (ALTER EXTENSION UPDATE); "upgrade" is
+# cluster-level (pg_upgrade) and is a separate, not-yet-implemented axis. Don't
+# conflate the two in variable names or comments.
+#
+# The mode is signalled to test/deps.sql via the count_nulls.test_load_mode
+# placeholder GUC. pg_regress does not forward make variables, but the psql
+# processes it spawns inherit the environment, so PGOPTIONS reaches deps.sql.
+# It's exported unconditionally so deps.sql can read it without missing_ok
+# and fail loudly if it didn't propagate, instead of silently defaulting to
+# fresh and running the wrong suite.
+TEST_LOAD_SOURCE ?= fresh
+ifeq ($(filter $(TEST_LOAD_SOURCE),fresh update),)
+$(error TEST_LOAD_SOURCE must be 'fresh' or 'update', 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/build/expected/upgrade.out b/test/build/expected/upgrade.out
new file mode 100644
index 0000000..25fdbb1
--- /dev/null
+++ b/test/build/expected/upgrade.out
@@ -0,0 +1 @@
+\set ECHO none
diff --git a/test/build/upgrade.sql b/test/build/upgrade.sql
new file mode 100644
index 0000000..c5a5946
--- /dev/null
+++ b/test/build/upgrade.sql
@@ -0,0 +1,11 @@
+\set ECHO none
+\i test/pgxntool/psql.sql
+\t
+
+-- Sanity check: install the oldest available version and upgrade to current.
+-- sql/count_nulls--0.9.6.sql is the oldest full install script we still ship
+-- (0.9.0/0.9.2/0.9.5 only exist as upgrade diffs, not standalone installs).
+BEGIN;
+CREATE EXTENSION count_nulls VERSION '0.9.6';
+ALTER EXTENSION count_nulls UPDATE;
+ROLLBACK;
diff --git a/test/deps.sql b/test/deps.sql
index 9705f9f..454be4b 100644
--- a/test/deps.sql
+++ b/test/deps.sql
@@ -5,4 +5,45 @@ CREATE SCHEMA IF NOT EXISTS :schema;
 SET search_path = :schema;
 SET client_min_messages = NOTICE;
 
+/*
+ * 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. Since every test file loads this via
+ * test/load.sql, running the suite in each mode (make test / make
+ * test-update) exercises the SAME tests against a fresh vs. an updated
+ * install, with the same expected output either way.
+ *
+ * "update" here is extension-level (ALTER EXTENSION UPDATE); it is not
+ * "upgrade" (cluster-level pg_upgrade), a separate axis this doesn't cover.
+ *
+ * The Makefile always exports this GUC, so we read it without missing_ok:
+ * if it failed to propagate we want a loud error here, not a silent
+ * fall-through to 'fresh'.
+ */
+SELECT current_setting('count_nulls.test_load_mode') AS count_nulls_test_load_mode
+\gset
+
+DO $$
+BEGIN
+  IF current_setting('count_nulls.test_load_mode') NOT IN ('fresh', 'update') THEN
+    RAISE EXCEPTION
+      'count_nulls.test_load_mode must be ''fresh'' or ''update'', got ''%'''
+      , current_setting('count_nulls.test_load_mode')
+    ;
+  END IF;
+END
+$$;
+
+SELECT :'count_nulls_test_load_mode' = 'update' AS count_nulls_update_mode
+\gset
+
+\if :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