diff --git a/internal/database/postgres/migrations/000133_purchase_history_reverify_ec2_offering_class.down.sql b/internal/database/postgres/migrations/000133_purchase_history_reverify_ec2_offering_class.down.sql
new file mode 100644
index 00000000..79fc9836
--- /dev/null
+++ b/internal/database/postgres/migrations/000133_purchase_history_reverify_ec2_offering_class.down.sql
@@ -0,0 +1,6 @@
+-- Intentionally a no-op: the up migration cannot record which rows it cleared
+-- without also keeping the stale class it exists to remove, and re-stamping
+-- every NULL EC2 row 'convertible' would re-block Standard RIs (and overwrite
+-- pre-000087 rows that were never stamped). Cleared rows that were already
+-- re-fetched from AWS keep their authoritative class.
+SELECT 1;
diff --git a/internal/database/postgres/migrations/000133_purchase_history_reverify_ec2_offering_class.up.sql b/internal/database/postgres/migrations/000133_purchase_history_reverify_ec2_offering_class.up.sql
new file mode 100644
index 00000000..99d79cb1
--- /dev/null
+++ b/internal/database/postgres/migrations/000133_purchase_history_reverify_ec2_offering_class.up.sql
@@ -0,0 +1,28 @@
+-- Clear stale EC2 RI offering_class stamps so they are re-read from AWS
+-- (platform#524).
+--
+-- Before platform#509, savePurchaseHistory stamped every AWS 'ec2' row
+-- 'convertible' whatever global_config.offering_class said. Rows bought while
+-- that setting was 'standard' are Standard RIs recorded as convertible, so the
+-- marketplace-sell guard refuses them and the History page hides Sell.
+-- global_config keeps no history, so the database alone cannot tell those rows
+-- from genuinely Convertible ones; only AWS DescribeReservedInstances can.
+--
+-- Owner option (a) on the issue: drop the convertible stamp on every AWS EC2
+-- RI row (both the legacy 'ec2' and the canonical 'compute' slug, which also
+-- clears any post-#509 re-drive stamp that recorded the class configured at
+-- retry time). NULL is the column's "unknown" state: the marketplace-list
+-- handler then calls populateOfferingClass, which fetches the class from AWS
+-- and re-stamps the row, so a Standard RI becomes sellable and a Convertible
+-- one is stamped convertible again after a single refused (400) Sell attempt.
+--
+-- Only 'convertible' is cleared. A 'standard' stamp is left alone (this issue
+-- is about Standard RIs being blocked, not the reverse), and a concurrent
+-- StampOfferingClass that commits 'standard' first is not overwritten:
+-- under READ COMMITTED the UPDATE re-checks the WHERE clause on the
+-- committed row version and skips it.
+UPDATE purchase_history
+SET offering_class = NULL
+WHERE provider = 'aws'
+  AND service IN ('ec2', 'compute')
+  AND LOWER(offering_class) = 'convertible';
diff --git a/internal/database/postgres/migrations/000133_purchase_history_reverify_ec2_offering_class_test.go b/internal/database/postgres/migrations/000133_purchase_history_reverify_ec2_offering_class_test.go
new file mode 100644
index 00000000..d7d9215a
--- /dev/null
+++ b/internal/database/postgres/migrations/000133_purchase_history_reverify_ec2_offering_class_test.go
@@ -0,0 +1,135 @@
+//go:build integration
+// +build integration
+
+package migrations_test
+
+import (
+	"context"
+	"testing"
+	"time"
+
+	"github.com/LeanerCloud/cloud-commitments-platform/internal/database/postgres/migrations"
+	"github.com/LeanerCloud/cloud-commitments-platform/internal/database/postgres/testhelpers"
+	"github.com/jackc/pgx/v5/pgxpool"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+const migration133Version = 133
+
+// insertOfferingClassRow inserts one purchase_history row with the given
+// provider, service and offering_class (nil stores NULL).
+func insertOfferingClassRow(ctx context.Context, t *testing.T, pool *pgxpool.Pool, purchaseID, provider, service string, offeringClass *string) {
+	t.Helper()
+	_, err := pool.Exec(ctx, `
+		INSERT INTO purchase_history (
+			account_id, purchase_id, timestamp, provider, service, region,
+			resource_type, term, payment, offering_class
+		) VALUES ('test-account', $1, $2, $3, $4, 'us-east-1', 'm5.large', 1, 'all-upfront', $5)`,
+		purchaseID, time.Now(), provider, service, offeringClass)
+	require.NoError(t, err)
+}
+
+// offeringClassOf returns the row's offering_class, nil when NULL.
+func offeringClassOf(ctx context.Context, t *testing.T, pool *pgxpool.Pool, purchaseID string) *string {
+	t.Helper()
+	var class *string
+	require.NoError(t, pool.QueryRow(ctx,
+		`SELECT offering_class FROM purchase_history WHERE purchase_id = $1`, purchaseID).Scan(&class))
+	return class
+}
+
+// TestMigration133_ClearsConvertibleEC2Stamps locks down migration 000133
+// (issue #524): before #509 every AWS 'ec2' purchase_history row was stamped
+// 'convertible' even when the class setting was 'standard', which blocks
+// selling a Standard RI. The migration clears the convertible stamp on AWS EC2
+// rows (legacy 'ec2' and canonical 'compute' slugs) so the marketplace handler
+// re-fetches the class from AWS, and leaves every other row alone.
+//
+// Fail-before: with the UPDATE removed from the up file, the cleared rows keep
+// 'convertible' and the NULL assertions fail.
+func TestMigration133_ClearsConvertibleEC2Stamps(t *testing.T) {
+	ctx, cancel := context.WithTimeout(context.Background(), 3*time.Minute)
+	defer cancel()
+
+	container := testhelpers.RequirePostgresContainer(ctx, t)
+	defer container.Cleanup(ctx)
+	pool := container.DB.Pool()
+	migrationsPath := getMigrationsPath()
+
+	prev := previousMigrationVersion(t, migration133Version)
+	require.NoError(t, migrations.MigrateToVersion(ctx, pool, migrationsPath, prev))
+
+	cases := []struct {
+		purchaseID, provider, service string
+		before, after                 *string
+	}{
+		// The defect: a stale convertible stamp on an EC2 RI.
+		{"ri-ec2-convertible", "aws", "ec2", strPtr("convertible"), nil},
+		{"ri-compute-convertible", "aws", "compute", strPtr("convertible"), nil},
+		// The sell guard compares case-insensitively, so the reset does too.
+		{"ri-ec2-convertible-upper", "aws", "ec2", strPtr("Convertible"), nil},
+		// A standard stamp is already sellable and must not be touched.
+		{"ri-ec2-standard", "aws", "ec2", strPtr("standard"), strPtr("standard")},
+		{"ri-compute-standard", "aws", "compute", strPtr("standard"), strPtr("standard")},
+		// Already unknown: stays unknown.
+		{"ri-ec2-null", "aws", "ec2", nil, nil},
+		// Out of scope: other AWS services and other providers.
+		{"ri-rds-convertible", "aws", "rds", strPtr("convertible"), strPtr("convertible")},
+		{"azure-compute-convertible", "azure", "compute", strPtr("convertible"), strPtr("convertible")},
+	}
+	for _, c := range cases {
+		insertOfferingClassRow(ctx, t, pool, c.purchaseID, c.provider, c.service, c.before)
+	}
+
+	require.NoError(t, migrations.MigrateToVersion(ctx, pool, migrationsPath, migration133Version))
+	for _, c := range cases {
+		assert.Equal(t, c.after, offeringClassOf(ctx, t, pool, c.purchaseID), "after 000133: %s", c.purchaseID)
+	}
+
+	// The down migration is a no-op: it must apply cleanly and must not
+	// re-stamp cleared rows convertible (that would re-block Standard RIs).
+	require.NoError(t, migrations.MigrateToVersion(ctx, pool, migrationsPath, prev))
+	for _, c := range cases {
+		assert.Equal(t, c.after, offeringClassOf(ctx, t, pool, c.purchaseID), "after rollback: %s", c.purchaseID)
+	}
+}
+
+// TestMigration133_ConcurrentStandardStampWins covers the marketplace handler
+// stamping a row 'standard' from AWS (StampOfferingClass) while the migration
+// runs. The migration queues behind the stamp's row lock; once the stamp
+// commits, the UPDATE re-checks its WHERE clause on the new row version and
+// must leave the authoritative 'standard' in place rather than clearing it.
+func TestMigration133_ConcurrentStandardStampWins(t *testing.T) {
+	ctx, cancel := context.WithTimeout(context.Background(), 3*time.Minute)
+	defer cancel()
+
+	container := testhelpers.RequirePostgresContainer(ctx, t)
+	defer container.Cleanup(ctx)
+	pool := container.DB.Pool()
+	migrationsPath := getMigrationsPath()
+
+	prev := previousMigrationVersion(t, migration133Version)
+	require.NoError(t, migrations.MigrateToVersion(ctx, pool, migrationsPath, prev))
+
+	const purchaseID = "ri-ec2-race"
+	insertOfferingClassRow(ctx, t, pool, purchaseID, "aws", "ec2", strPtr("convertible"))
+
+	stamp, err := pool.Begin(ctx)
+	require.NoError(t, err)
+	defer func() { _ = stamp.Rollback(ctx) }()
+	_, err = stamp.Exec(ctx, `UPDATE purchase_history SET offering_class = $1 WHERE purchase_id = $2`, "standard", purchaseID)
+	require.NoError(t, err)
+
+	done := make(chan error, 1)
+	go func() { done <- migrations.MigrateToVersion(ctx, pool, migrationsPath, migration133Version) }()
+
+	// pg_stat_activity truncates query text at 1024 bytes, which cuts the
+	// migration file inside its header comment, so match on that comment.
+	testhelpers.WaitForLockWait(ctx, t, pool, "%stale EC2 RI offering_class%")
+	require.NoError(t, stamp.Commit(ctx))
+	require.NoError(t, testhelpers.Await(t, done))
+
+	assert.Equal(t, strPtr("standard"), offeringClassOf(ctx, t, pool, purchaseID),
+		"a standard stamp committed while 000133 waited must survive the migration")
+}