fix(billing): a scheduled downgrade must be applicable, and never cost money

Once the term is over, the downgrade has to be allowed — otherwise the job that
is supposed to carry it out never can, and the change waits forever for a date
that has already passed.

And a grandfathered plan can be cheaper than the smaller plan costs today, which
made the goodwill credit negative: an invoice for the privilege of downgrading.
Clamped to zero.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

app/Services/Billing/PlanChange.php

@@ -50,13 +50,17 @@ final readonly class PlanChange
         $remainingDays = (int) max(0, floor($at->diffInDays($subscription->current_period_end, absolute: false)));
 
         if (! $isUpgrade) {
-            // Downgrade: nothing changes and nothing is owed until the term ends.
+            // Downgrade: nothing changes and nothing is owed until the term
+            // ends — but once it HAS ended it must actually be applicable, or
+            // the job that is supposed to carry it out never can.
+            $termOver = $at->greaterThanOrEqualTo($subscription->current_period_end);
+
             return new self(
                 isUpgrade: false,
-                allowedNow: false,
+                allowedNow: $termOver,
                 chargeCents: 0,
                 creditCents: 0,
-                effectiveAt: $subscription->current_period_end->copy(),
+                effectiveAt: $termOver ? $at->copy() : $subscription->current_period_end->copy(),
                 remainingDays: $remainingDays,
                 termDays: $termDays,
             );
@@ -100,8 +104,9 @@ final readonly class PlanChange
         $target = Subscription::snapshotFrom($targetPlan, $subscription->term);
         $difference = $subscription->price_cents - $target['price_cents'];
 
-        // Only the unused part of the difference: they used the expensive plan
+        // Never negative: a grandfathered plan can be cheaper than the smaller
-        // for the days that have passed.
+        // plan costs today, and a downgrade must not invoice the customer for
-        return (int) round($difference * $change->remainingDays / $change->termDays);
+        // the privilege.
+        return max(0, (int) round($difference * $change->remainingDays / $change->termDays));
     }
 }

tests/Feature/Billing/PlanChangeTest.php

@@ -144,3 +144,37 @@ it('reads the direction from the plan rank, not from a grandfathered price', fun
         ->and($change->allowedNow)->toBeFalse()
         ->and($change->chargeCents)->toBe(0);
 });
+
+it('lets a scheduled downgrade actually happen once the term is over', function () {
+    $start = Illuminate\Support\Carbon::parse('2026-08-01');
+    $subscription = Subscription::factory()->plan('business')->create([
+        'current_period_start' => $start,
+        'current_period_end' => $start->copy()->addMonth(),
+    ]);
+
+    // Mid-term: not yet.
+    expect(PlanChange::evaluate($subscription, 'team', Illuminate\Support\Carbon::parse('2026-08-15'))->allowedNow)
+        ->toBeFalse();
+
+    // On the boundary and after it: the job carrying it out has to be able to.
+    foreach (['2026-09-01', '2026-09-03'] as $when) {
+        $change = PlanChange::evaluate($subscription, 'team', Illuminate\Support\Carbon::parse($when));
+        expect($change->allowedNow)->toBeTrue()
+            ->and($change->effectiveAt->toDateString())->toBe($when);
+    }
+});
+
+it('never invoices a customer for downgrading', function () {
+    $start = Illuminate\Support\Carbon::parse('2026-08-01');
+    $subscription = Subscription::factory()->plan('business')->create([
+        'current_period_start' => $start,
+        'current_period_end' => $start->copy()->addMonth(),
+    ]);
+    // Grandfathered below today's smaller plan.
+    $subscription->forceFill(['price_cents' => 9900])->saveQuietly();
+    config()->set('provisioning.plans.team.price_cents', 17900);
+
+    $credit = PlanChange::goodwillCredit($subscription->fresh(), 'team', Illuminate\Support\Carbon::parse('2026-08-15'));
+
+    expect($credit)->toBe(0);
+});