diff --git a/server/src/internal/billing/v2/actions/attach/setup/setupAttachTransitionContext.ts b/server/src/internal/billing/v2/actions/attach/setup/setupAttachTransitionContext.ts
index ae16a64c1..16601efcb 100644
--- a/server/src/internal/billing/v2/actions/attach/setup/setupAttachTransitionContext.ts
+++ b/server/src/internal/billing/v2/actions/attach/setup/setupAttachTransitionContext.ts
@@ -5,6 +5,8 @@ import {
 	type FullProduct,
 	findMainActiveCustomerProductByGroup,
 	findMainScheduledCustomerProductByGroup,
+	getProductBaseInterval,
+	intervalsDifferent,
 	isOneOffProduct,
 	isProductUpgrade,
 } from "@autumn/shared";
@@ -53,12 +55,30 @@ export const setupAttachTransitionContext = ({
 			cusProduct: currentCustomerProduct,
 		});
 
-		const isUpgrade = isProductUpgrade({
-			prices1: currentPrices,
-			prices2: attachProduct.prices,
+		const currentBaseInterval = getProductBaseInterval({
+			prices: currentPrices,
+		});
+		const newBaseInterval = getProductBaseInterval({
+			prices: attachProduct.prices,
 		});
 
-		planTiming = isUpgrade ? "immediate" : "end_of_cycle";
+		const baseIntervalsAreDifferent =
+			currentBaseInterval &&
+			newBaseInterval &&
+			intervalsDifferent({
+				intervalA: currentBaseInterval,
+				intervalB: newBaseInterval,
+			});
+
+		if (baseIntervalsAreDifferent) {
+			planTiming = "immediate";
+		} else {
+			const isUpgrade = isProductUpgrade({
+				prices1: currentPrices,
+				prices2: attachProduct.prices,
+			});
+			planTiming = isUpgrade ? "immediate" : "end_of_cycle";
+		}
 	}
 
 	// Override if plan_schedule param is provided
diff --git a/server/tests/integration/billing/attach/immediate-switch/immediate-switch-basic.test.ts b/server/tests/integration/billing/attach/immediate-switch/immediate-switch-basic.test.ts
index 549deb617..8a20ea33d 100644
--- a/server/tests/integration/billing/attach/immediate-switch/immediate-switch-basic.test.ts
+++ b/server/tests/integration/billing/attach/immediate-switch/immediate-switch-basic.test.ts
@@ -556,3 +556,89 @@ test.concurrent(`${chalk.yellowBright("immediate-switch-basic 6: invoice line it
 		notPresent: [pro.id],
 	});
 });
+
+// ═══════════════════════════════════════════════════════════════════════════════
+// TEST 7: Pro Annual to Premium Monthly (interval change = immediate)
+// ═══════════════════════════════════════════════════════════════════════════════
+
+/**
+ * Scenario:
+ * - Customer has pro annual ($200/year)
+ * - Switch to premium monthly ($50/mo)
+ *
+ * Expected Result:
+ * - Premium is active immediately (different billing interval = always immediate)
+ * - Pro annual is removed
+ * - Credit for unused annual applied against premium charge
+ */
+test.concurrent(`${chalk.yellowBright("immediate-switch-basic 7: pro annual to premium monthly (interval change)")}`, async () => {
+	const customerId = "imm-switch-pro-annual-to-premium";
+
+	const proAnnualMessages = items.monthlyMessages({ includedUsage: 500 });
+	const proAnnual = products.proAnnual({
+		id: "pro-annual",
+		items: [proAnnualMessages],
+	});
+
+	const premiumMessages = items.monthlyMessages({ includedUsage: 1000 });
+	const premium = products.premium({
+		id: "premium",
+		items: [premiumMessages],
+	});
+
+	const { autumnV1, ctx } = await initScenario({
+		customerId,
+		setup: [
+			s.customer({ paymentMethod: "success" }),
+			s.products({ list: [proAnnual, premium] }),
+		],
+		actions: [s.billing.attach({ productId: proAnnual.id })],
+	});
+
+	// 1. Preview switch to premium monthly
+	// At start of cycle: credit for full annual ($200) exceeds premium monthly ($50)
+	const preview = await autumnV1.billing.previewAttach({
+		customer_id: customerId,
+		product_id: premium.id,
+	});
+	expect(preview.total).toBe(0);
+
+	// 2. Attach premium (immediate because interval differs: annual -> monthly)
+	await autumnV1.billing.attach({
+		customer_id: customerId,
+		product_id: premium.id,
+		redirect_mode: "if_required",
+	});
+
+	const customer = await autumnV1.customers.get<ApiCustomerV3>(customerId);
+
+	// Premium should be active, pro annual should be gone (immediate switch)
+	await expectCustomerProducts({
+		customer,
+		active: [premium.id],
+		notPresent: [proAnnual.id],
+	});
+
+	// Verify messages feature has premium's balance
+	expectCustomerFeatureCorrect({
+		customer,
+		featureId: TestFeature.Messages,
+		includedUsage: 1000,
+		balance: 1000,
+		usage: 0,
+	});
+
+	// Verify invoices: proAnnual ($200) + switch invoice ($0, credit exceeds charge)
+	await expectCustomerInvoiceCorrect({
+		customer,
+		count: 2,
+		latestTotal: 0,
+	});
+
+	await expectSubToBeCorrect({
+		db: ctx.db,
+		customerId,
+		org: ctx.org,
+		env: ctx.env,
+	});
+});
diff --git a/server/tests/integration/billing/attach/scheduled-switch/scheduled-switch-entities-cross.test.ts b/server/tests/integration/billing/attach/scheduled-switch/scheduled-switch-entities-cross.test.ts
index 7a65c6250..f1e5b8320 100644
--- a/server/tests/integration/billing/attach/scheduled-switch/scheduled-switch-entities-cross.test.ts
+++ b/server/tests/integration/billing/attach/scheduled-switch/scheduled-switch-entities-cross.test.ts
@@ -344,13 +344,15 @@ test.concurrent(`${chalk.yellowBright("scheduled-switch-entities-cross 3: entity
 
 /**
  * Scenario:
- * - Entity 1: Premium Annual → Pro (scheduled)
- * - Entity 2: Premium Monthly → Pro (scheduled)
+ * - Entity 1: Premium Annual → Pro (immediate, interval change)
+ * - Entity 2: Premium Monthly → Pro (scheduled, same interval)
  * - Advance 1 month (monthly cycle ends)
  * - Upgrade entity 2 back to premium
  *
  * Expected Result:
- * - After cycle: Entity 1 still on annual (hasn't ended yet), Entity 2 on pro
+ * - Entity 1 immediately switches to pro (annual→monthly = different interval = immediate)
+ * - Entity 2 has premium canceling, pro scheduled (same interval = end_of_cycle)
+ * - After cycle: Entity 1 on pro (renewed), Entity 2 on pro (scheduled switch completed)
  * - After upgrade: Entity 2 on premium
  */
 test.concurrent(`${chalk.yellowBright("scheduled-switch-entities-cross 4: entity 1 premiumAnnual to pro, entity 2 premium to pro, advance cycle, upgrade entity 2 to premium")}`, async () => {
@@ -385,28 +387,27 @@ test.concurrent(`${chalk.yellowBright("scheduled-switch-entities-cross 4: entity
 		actions: [
 			s.billing.attach({ productId: premiumAnnual.id, entityIndex: 0 }),
 			s.billing.attach({ productId: premium.id, entityIndex: 1 }),
-			s.billing.attach({ productId: pro.id, entityIndex: 0 }), // Downgrade entity 1 (annual)
-			s.billing.attach({ productId: pro.id, entityIndex: 1 }), // Downgrade entity 2 (monthly)
+			s.billing.attach({ productId: pro.id, entityIndex: 0 }), // Entity 1: immediate (interval change)
+			s.billing.attach({ productId: pro.id, entityIndex: 1 }), // Entity 2: scheduled (same interval)
 			s.advanceToNextInvoice(), // Advance 1 month
 		],
 	});
 
-	// Verify entity 1: still on annual (annual hasn't ended)
+	// Entity 1 already switched to pro immediately (annual→monthly = different interval)
 	const entity1Before = await autumnV1.entities.get<ApiEntityV0>(
 		customerId,
 		entities[0].id,
 	);
-	// Entity 1's annual subscription should still be active with pro scheduled
-	await expectProductCanceling({
-		customer: entity1Before,
-		productId: premiumAnnual.id,
-	});
-	await expectProductScheduled({
+	await expectProductActive({
 		customer: entity1Before,
 		productId: pro.id,
 	});
+	await expectProductNotPresent({
+		customer: entity1Before,
+		productId: premiumAnnual.id,
+	});
 
-	// Verify entity 2: now on pro (monthly cycle completed)
+	// Entity 2: now on pro (monthly cycle completed, scheduled switch took effect)
 	const entity2Before = await autumnV1.entities.get<ApiEntityV0>(
 		customerId,
 		entities[1].id,
diff --git a/shared/utils/productUtils/priceUtils/classifyPriceUtils.ts b/shared/utils/productUtils/priceUtils/classifyPriceUtils.ts
index 8ab19cf2d..32f98683d 100644
--- a/shared/utils/productUtils/priceUtils/classifyPriceUtils.ts
+++ b/shared/utils/productUtils/priceUtils/classifyPriceUtils.ts
@@ -11,6 +11,16 @@ import { BillingType } from "../../../models/productModels/priceModels/priceEnum
 import type { Price } from "../../../models/productModels/priceModels/priceModels";
 import { getBillingType } from "../priceUtils";
 
+/** Returns the billing interval of the base (fixed) price for a set of prices, or null if none found. */
+export const getProductBaseInterval = ({ prices }: { prices: Price[] }) => {
+	const basePrice = prices.find(isFixedPrice);
+	if (!basePrice) return null;
+	return {
+		interval: basePrice.config.interval as BillingInterval,
+		intervalCount: basePrice.config.interval_count ?? 1,
+	};
+};
+
 export const isOneOffPrice = (
 	price: Price,
 ): price is Price & {