billing: Allow legacy servers to cancel upgrade.

corporate/lib/stripe.py

@@ -1869,7 +1869,14 @@ class BillingSession(ABC):
         if status is not None:
             if status == CustomerPlan.ACTIVE:
                 assert plan.status < CustomerPlan.LIVE_STATUS_THRESHOLD
-                do_change_plan_status(plan, status)
+                with transaction.atomic():  # nocoverage
+                    # Switch to a different plan was cancelled. We end the next plan
+                    # and set the current one as active.
+                    if plan.status == CustomerPlan.SWITCH_PLAN_TIER_AT_PLAN_END:
+                        next_plan = self.get_next_plan(plan)
+                        assert next_plan is not None
+                        do_change_plan_status(next_plan, CustomerPlan.ENDED)
+                    do_change_plan_status(plan, status)
             elif status == CustomerPlan.DOWNGRADE_AT_END_OF_CYCLE:
                 assert not plan.is_free_trial()
                 assert plan.status < CustomerPlan.LIVE_STATUS_THRESHOLD

corporate/views/billing_page.py

@@ -169,9 +169,12 @@ def remote_server_billing_page(
         context["sponsorship_pending"] = True
 
     if (
-        billing_session.remote_server.plan_type == RemoteZulipServer.PLAN_TYPE_SELF_HOSTED
+        customer is None
-        or customer is None
         or get_current_plan_by_customer(customer) is None
+        or (
+            billing_session.get_legacy_remote_server_new_plan_name(customer) is None
+            and billing_session.remote_server.plan_type == RemoteZulipServer.PLAN_TYPE_SELF_HOSTED
+        )
     ):
         return HttpResponseRedirect(
             reverse(

templates/corporate/billing.html

@@ -252,7 +252,7 @@
                     <input name="status" type="hidden" value="{{ CustomerPlan.DOWNGRADE_AT_END_OF_FREE_TRIAL }}" />
                     {% elif downgrade_at_end_of_free_trial %}
                     <input name="status" type="hidden" value="{{ CustomerPlan.FREE_TRIAL }}" />
-                    {% elif downgrade_at_end_of_cycle %}
+                    {% elif downgrade_at_end_of_cycle or is_server_on_legacy_plan %}
                     <input name="status" type="hidden" value="{{ CustomerPlan.ACTIVE }}" />
                     {% else %}
                     <input name="status" type="hidden" value="{{ CustomerPlan.DOWNGRADE_AT_END_OF_CYCLE }}" />
@@ -392,4 +392,27 @@
         </div>
     </div>
 </div>
+<div id="confirm-cancel-legacy-server-upgrade-modal" class="micromodal" aria-hidden="true">
+    <div class="modal__overlay" tabindex="-1">
+        <div class="modal__container" role="dialog" aria-modal="true" aria-labelledby="dialog_title">
+            <header class="modal__header">
+                <h1 class="modal__title dialog_heading">
+                    Downgrade?
+                </h1>
+                <button class="modal__close" aria-label="{{ _('Close modal') }}" data-micromodal-close></button>
+            </header>
+            <main class="modal__content">
+                <p>
+                    Lose benefits?
+                </p>
+            </main>
+            <footer class="modal__footer">
+                <button class="modal__btn dialog_exit_button" aria-label="{{ '(Close this dialog window)' }}" data-micromodal-close>{{ _('Never mind') }}</button>
+                <button class="modal__btn dialog_submit_button">
+                    <span>{{ _('Downgrade') }}</span>
+                </button>
+            </footer>
+        </div>
+    </div>
+</div>
 {% endblock %}

web/src/billing/billing.ts

@@ -173,6 +173,26 @@ export function initialize(): void {
         portico_modals.open($modal.attr("id")!);
     });
 
+    $("#cancel-legacy-server-upgrade").on("click", (e) => {
+        e.preventDefault();
+        portico_modals.open("confirm-cancel-legacy-server-upgrade-modal");
+    });
+
+    $("#confirm-cancel-legacy-server-upgrade-modal .dialog_submit_button").on("click", (e) => {
+        helpers.create_ajax_request(
+            `/json${billing_base_url}/billing/plan`,
+            "planchange",
+            [],
+            "PATCH",
+            () =>
+                window.location.replace(
+                    `${billing_base_url}/upgrade/?success_message=` +
+                        encodeURIComponent("Your plan is no longer scheduled for an upgrade."),
+                ),
+        );
+        e.preventDefault();
+    });
+
     $("#confirm-licenses-modal-increase, #confirm-licenses-modal-decrease").on(
         "click",
         ".dialog_submit_button",