diff --git a/packages/functional-tests/pages/payments/index.ts b/packages/functional-tests/pages/payments/index.ts
index 012f6a98318..a7e754ab74f 100644
--- a/packages/functional-tests/pages/payments/index.ts
+++ b/packages/functional-tests/pages/payments/index.ts
@@ -5,9 +5,11 @@
 import { Page } from '@playwright/test';
 import { BaseTarget } from '../../lib/targets/base';
 import { CheckoutPage } from './checkout';
+import { UpgradePage } from './upgrade';
 
 export function create(page: Page, target: BaseTarget) {
   return {
     checkout: new CheckoutPage(page, target),
+    upgrade: new UpgradePage(page, target),
   };
 }
diff --git a/packages/functional-tests/pages/payments/upgrade.ts b/packages/functional-tests/pages/payments/upgrade.ts
new file mode 100644
index 00000000000..bfa1561303d
--- /dev/null
+++ b/packages/functional-tests/pages/payments/upgrade.ts
@@ -0,0 +1,114 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+import { Page, expect } from '@playwright/test';
+import { BaseTarget } from '../../lib/targets/base';
+
+export class UpgradePage {
+  constructor(
+    public page: Page,
+    readonly target: BaseTarget
+  ) {}
+
+  // Start page — upgrade section
+
+  get upgradeSection() {
+    return this.page.getByTestId('subscription-upgrade');
+  }
+
+  get proratedAmount() {
+    return this.page.getByTestId('amount-due');
+  }
+
+  get acknowledgmentText() {
+    return this.page.getByTestId('sub-update-acknowledgment');
+  }
+
+  // Consent and submit (shared with checkout via CheckoutForm)
+
+  get consentCheckbox() {
+    return this.page.locator(
+      'input[type="checkbox"][name="confirm"][aria-required]'
+    );
+  }
+
+  get confirmUpgradeButton() {
+    return this.page.getByRole('button', { name: /Subscribe Now/i });
+  }
+
+  // Success page
+
+  get successHeading() {
+    return this.page.locator('#subscription-confirmation-heading');
+  }
+
+  get invoiceNumber() {
+    return this.page.getByText(/Invoice #/);
+  }
+
+  get successActionButton() {
+    return this.page
+      .locator('section[aria-labelledby="subscription-confirmation-heading"]')
+      .locator('a[role="button"]');
+  }
+
+  // Error page (checkout error page handles eligibility errors)
+
+  get eligibilityError() {
+    return this.page.locator('#page-information-heading');
+  }
+
+  get errorButton() {
+    return this.page.getByRole('link', {
+      name: /Contact Support|Manage my subscription|Try again/i,
+    });
+  }
+
+  // Actions
+
+  /**
+   * Check the consent checkbox for upgrade. Waits for the checkbox to
+   * become interactive before clicking.
+   */
+  async checkConsent() {
+    await expect(this.consentCheckbox).toBeVisible({ timeout: 10_000 });
+    await expect(this.consentCheckbox).not.toHaveAttribute(
+      'aria-disabled',
+      'true',
+      { timeout: 10_000 }
+    );
+    await this.consentCheckbox.click();
+    await expect(this.consentCheckbox).toBeChecked();
+  }
+
+  /**
+   * Click the Subscribe Now button to confirm upgrade. Waits for it to
+   * be enabled (aria-disabled="false") before clicking.
+   */
+  async confirmUpgrade() {
+    await expect(this.confirmUpgradeButton).toHaveAttribute(
+      'aria-disabled',
+      'false',
+      { timeout: 30_000 }
+    );
+    await this.confirmUpgradeButton.click();
+  }
+
+  /**
+   * Wait for the upgrade success page heading to become visible.
+   */
+  async waitForSuccess(timeout = 60_000) {
+    await expect(this.page).toHaveURL(/success/, { timeout });
+    await expect(this.successHeading).toBeVisible({ timeout: 30_000 });
+  }
+
+  /**
+   * Wait for the eligibility error page to become visible.
+   * Used for duplicate subscription and downgrade blocked scenarios.
+   */
+  async waitForEligibilityError(timeout = 60_000) {
+    await expect(this.page).toHaveURL(/error/, { timeout });
+    await expect(this.eligibilityError).toBeVisible({ timeout: 10_000 });
+  }
+}
diff --git a/packages/functional-tests/tests-payments-next/upgrade.spec.ts b/packages/functional-tests/tests-payments-next/upgrade.spec.ts
new file mode 100644
index 00000000000..e352ec5da1c
--- /dev/null
+++ b/packages/functional-tests/tests-payments-next/upgrade.spec.ts
@@ -0,0 +1,175 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+import { expect, test } from '../lib/fixtures/payments';
+import { StripeTestCards } from '../lib/stripe-test-cards';
+
+// Upgrade tests are resource-intensive (require an existing subscription,
+// then trigger the upgrade flow). Run serially to avoid overwhelming
+// local services and Stripe rate limits.
+test.describe.configure({ mode: 'serial' });
+
+test.describe('severity-1 #smoke', () => {
+  test.setTimeout(240_000);
+  test.use({ viewport: { width: 1280, height: 1080 } });
+
+  test.beforeEach(async ({}, { project }) => {
+    test.skip(
+      project.name.includes('production'),
+      'Upgrade smoke tests are not run in production'
+    );
+  });
+
+  test('Upgrade happy path — lower-tier to higher-tier with prorated price', async ({
+    target,
+    page,
+    pages: { relier, signin },
+    paymentPages: { checkout, upgrade },
+    testAccountTracker,
+  }) => {
+    const credentials = await testAccountTracker.signUp();
+
+    // Step 1: Subscribe to the monthly (lower-tier) plan
+    await relier.goto();
+    await relier.clickSubscribeMonthly();
+
+    await checkout.handleLocationIfNeeded();
+
+    await checkout.emailInput.fill(credentials.email);
+    await checkout.signInContinueButton.click();
+
+    await expect(page).toHaveURL(new RegExp(target.contentServerUrl), {
+      timeout: 30_000,
+    });
+    await signin.fillOutPasswordForm(credentials.password);
+
+    await expect(checkout.paymentHeading).toBeVisible({ timeout: 30_000 });
+
+    await checkout.waitForStripeReady();
+    await checkout.checkConsent();
+    await checkout.fillCard(StripeTestCards.SUCCESS);
+    await checkout.submit();
+
+    await checkout.waitForSuccess();
+    await expect(checkout.successHeading).toContainText('check your email');
+
+    // Step 2: Navigate to the 12-month (higher-tier) plan to trigger upgrade
+    await relier.goto();
+    await relier.clickSubscribe12Month();
+
+    // The system detects the existing subscription and redirects to the
+    // upgrade flow instead of a new checkout
+    await expect(upgrade.upgradeSection).toBeVisible({ timeout: 60_000 });
+    await expect(upgrade.proratedAmount).toBeVisible();
+    await expect(upgrade.acknowledgmentText).toBeVisible();
+
+    // Confirm the upgrade
+    await upgrade.checkConsent();
+    await upgrade.confirmUpgrade();
+
+    // Verify success
+    await upgrade.waitForSuccess();
+    await expect(upgrade.successHeading).toBeVisible();
+  });
+});
+
+test.describe('severity-2', () => {
+  test.setTimeout(240_000);
+  test.use({ viewport: { width: 1280, height: 1080 } });
+
+  test.beforeEach(async ({}, { project }) => {
+    test.skip(
+      project.name.includes('production'),
+      'Upgrade tests are not run in production'
+    );
+  });
+
+  test('Duplicate subscription blocked — already subscribed user is rejected', async ({
+    target,
+    page,
+    pages: { relier, signin },
+    paymentPages: { checkout, upgrade },
+    testAccountTracker,
+  }) => {
+    const credentials = await testAccountTracker.signUp();
+
+    // Step 1: Complete initial monthly subscription
+    await relier.goto();
+    await relier.clickSubscribeMonthly();
+
+    await checkout.handleLocationIfNeeded();
+
+    await checkout.emailInput.fill(credentials.email);
+    await checkout.signInContinueButton.click();
+
+    await expect(page).toHaveURL(new RegExp(target.contentServerUrl), {
+      timeout: 30_000,
+    });
+    await signin.fillOutPasswordForm(credentials.password);
+
+    await expect(checkout.paymentHeading).toBeVisible({ timeout: 30_000 });
+
+    await checkout.waitForStripeReady();
+    await checkout.checkConsent();
+    await checkout.fillCard(StripeTestCards.SUCCESS);
+    await checkout.submit();
+
+    await checkout.waitForSuccess();
+
+    // Step 2: Attempt to subscribe to the same monthly plan again
+    await relier.goto();
+    await relier.clickSubscribeMonthly();
+
+    // The system detects the duplicate subscription and shows an error
+    await upgrade.waitForEligibilityError();
+    await expect(upgrade.eligibilityError).toContainText(
+      /already subscribed/i
+    );
+    await expect(upgrade.errorButton).toBeVisible();
+  });
+
+  test('Downgrade blocked — higher-tier user cannot subscribe to lower tier', async ({
+    target,
+    page,
+    pages: { relier, signin },
+    paymentPages: { checkout, upgrade },
+    testAccountTracker,
+  }) => {
+    const credentials = await testAccountTracker.signUp();
+
+    // Step 1: Complete initial 12-month (higher-tier) subscription
+    await relier.goto();
+    await relier.clickSubscribe12Month();
+
+    await checkout.handleLocationIfNeeded();
+
+    await checkout.emailInput.fill(credentials.email);
+    await checkout.signInContinueButton.click();
+
+    await expect(page).toHaveURL(new RegExp(target.contentServerUrl), {
+      timeout: 30_000,
+    });
+    await signin.fillOutPasswordForm(credentials.password);
+
+    await expect(checkout.paymentHeading).toBeVisible({ timeout: 30_000 });
+
+    await checkout.waitForStripeReady();
+    await checkout.checkConsent();
+    await checkout.fillCard(StripeTestCards.SUCCESS);
+    await checkout.submit();
+
+    await checkout.waitForSuccess();
+
+    // Step 2: Attempt to subscribe to the monthly (lower-tier) plan
+    await relier.goto();
+    await relier.clickSubscribeMonthly();
+
+    // The system detects the downgrade attempt and shows an error
+    await upgrade.waitForEligibilityError();
+    await expect(upgrade.eligibilityError).toContainText(
+      /contact support/i
+    );
+    await expect(upgrade.errorButton).toBeVisible();
+  });
+});