diff --git a/server/src/internal/billing/v2/subscriptionUpdate/compute/computeSubscriptionUpdateAllPlan.ts b/server/src/internal/billing/v2/subscriptionUpdate/compute/computeSubscriptionUpdateAllPlan.ts
index 133fd731d..f9da01593 100644
--- a/server/src/internal/billing/v2/subscriptionUpdate/compute/computeSubscriptionUpdateAllPlan.ts
+++ b/server/src/internal/billing/v2/subscriptionUpdate/compute/computeSubscriptionUpdateAllPlan.ts
@@ -7,3 +7,19 @@
 // 	newItems: body.items,
 // 	products: [product],
 // });
+
+import type { SubscriptionUpdateV0Params } from "../../../../../../../shared";
+import type { AutumnContext } from "../../../../../honoUtils/HonoEnv";
+import type { UpdateSubscriptionContext } from "../fetch/updateSubscriptionContextSchema";
+
+export const computeSubscriptionUpdateCustomConfigurationPlan = ({
+	ctx: AutumnContext,
+	updateSubscriptionContext,
+	params,
+}: {
+	ctx: AutumnContext;
+	updateSubscriptionContext: UpdateSubscriptionContext;
+	params: SubscriptionUpdateV0Params;
+}) => {
+	return {};
+};
diff --git a/server/src/internal/billing/v2/subscriptionUpdate/compute/computeSubscriptionUpdatePlan.ts b/server/src/internal/billing/v2/subscriptionUpdate/compute/computeSubscriptionUpdatePlan.ts
index dd5ddc25b..2950595bc 100644
--- a/server/src/internal/billing/v2/subscriptionUpdate/compute/computeSubscriptionUpdatePlan.ts
+++ b/server/src/internal/billing/v2/subscriptionUpdate/compute/computeSubscriptionUpdatePlan.ts
@@ -22,6 +22,7 @@ export const computeSubscriptionUpdatePlan = (
 	},
 ): SubscriptionUpdatePlan => {
 	const intent = computeSubscriptionUpdateIntent(params);
+
 	const computePlan = getComputeSubscriptionUpdatePlanIntentMap(intent);
 
 	return computePlan(ctx, { updateSubscriptionContext, params });
diff --git a/server/src/internal/billing/v2/subscriptionUpdate/fetch/fetchApiSubscriptionUpdateContext.ts b/server/src/internal/billing/v2/subscriptionUpdate/fetch/fetchApiSubscriptionUpdateContext.ts
index a485bf452..ba19a9b6d 100644
--- a/server/src/internal/billing/v2/subscriptionUpdate/fetch/fetchApiSubscriptionUpdateContext.ts
+++ b/server/src/internal/billing/v2/subscriptionUpdate/fetch/fetchApiSubscriptionUpdateContext.ts
@@ -7,6 +7,7 @@ import type { AutumnContext } from "@/honoUtils/HonoEnv";
 import { CusService } from "../../../../customers/CusService";
 import { fetchStripeCustomerForBilling } from "../../fetch/fetchStripeUtils/fetchStripeCustomerForBilling";
 import { fetchStripeSubscriptionForBilling } from "../../fetch/fetchStripeUtils/fetchStripeSubscriptionForBilling";
+import { fetchTargetCusProductForUpdate } from "./fetchTargetCusProductForUpdate";
 import type { UpdateSubscriptionContext } from "./updateSubscriptionContextSchema";
 
 /**
@@ -42,9 +43,10 @@ export const fetchApiSubscriptionUpdateContext = async (
 		withEntities: true,
 	});
 
-	const targetCustomerProduct = fullCustomer.customer_products.find(
-		(cp) => cp.product.id === productId,
-	);
+	const targetCustomerProduct = fetchTargetCusProductForUpdate({
+		params,
+		fullCustomer,
+	});
 
 	if (!targetCustomerProduct) {
 		throw new InternalError({
diff --git a/server/src/internal/billing/v2/subscriptionUpdate/fetch/fetchTargetCusProductForUpdate.ts b/server/src/internal/billing/v2/subscriptionUpdate/fetch/fetchTargetCusProductForUpdate.ts
new file mode 100644
index 000000000..e098788ed
--- /dev/null
+++ b/server/src/internal/billing/v2/subscriptionUpdate/fetch/fetchTargetCusProductForUpdate.ts
@@ -0,0 +1,32 @@
+import {
+	type FullCustomer,
+	isCusProductOnEntity,
+	type SubscriptionUpdateV0Params,
+} from "@autumn/shared";
+
+export const fetchTargetCusProductForUpdate = ({
+	params,
+	fullCustomer,
+}: {
+	params: SubscriptionUpdateV0Params;
+	fullCustomer: FullCustomer;
+}) => {
+	const cusProducts = fullCustomer.customer_products;
+	const productId = params.product_id;
+	const internalEntityId = fullCustomer.entity?.internal_id;
+	const cusProductId = params.customer_product_id;
+
+	if (cusProductId) {
+		return cusProducts.find((cp) => cp.id === cusProductId);
+	}
+
+	return cusProducts.find((cp) => {
+		const productIdMatch = cp.product.id === productId;
+		const entityIdMatch = isCusProductOnEntity({
+			cusProduct: cp,
+			internalEntityId,
+		});
+
+		return productIdMatch && entityIdMatch;
+	});
+};
diff --git a/shared/api/billing/subscriptionUpdate/subscriptionUpdateV0Params.ts b/shared/api/billing/subscriptionUpdate/subscriptionUpdateV0Params.ts
index 4fc418665..b26a9ec82 100644
--- a/shared/api/billing/subscriptionUpdate/subscriptionUpdateV0Params.ts
+++ b/shared/api/billing/subscriptionUpdate/subscriptionUpdateV0Params.ts
@@ -4,7 +4,7 @@ import { ProductItemSchema } from "../../../models/productV2Models/productItemMo
 import { CustomerDataSchema } from "../../common/customerData";
 import { EntityDataSchema } from "../../models";
 
-export const SubscriptionUpdateV0ParamsSchema = z.object({
+export const ExtSubscriptionUpdateV0ParamsSchema = z.object({
 	// Customer / Entity Info
 	customer_id: z.string(),
 	product_id: z.string().nullish(),
@@ -26,6 +26,15 @@ export const SubscriptionUpdateV0ParamsSchema = z.object({
 	prorate_billing: z.boolean().optional(),
 });
 
+export const SubscriptionUpdateV0ParamsSchema =
+	ExtSubscriptionUpdateV0ParamsSchema.extend({
+		customer_product_id: z.string().optional(),
+	});
+
+export type ExtSubscriptionUpdateV0Params = z.infer<
+	typeof ExtSubscriptionUpdateV0ParamsSchema
+>;
+
 export type SubscriptionUpdateV0Params = z.infer<
 	typeof SubscriptionUpdateV0ParamsSchema
 >;
diff --git a/shared/utils/cusProductUtils/classifyCusProduct.ts b/shared/utils/cusProductUtils/classifyCusProduct.ts
index c7048021e..67ceeb1ad 100644
--- a/shared/utils/cusProductUtils/classifyCusProduct.ts
+++ b/shared/utils/cusProductUtils/classifyCusProduct.ts
@@ -43,7 +43,12 @@ export const isCusProductTrialing = ({
 	);
 };
 
-// ATTACH PRIMITIVES
+/**
+ * Returns true if the customer product is assigned to the given entity,
+ * or if no entity is specified, true if the product is not assigned to any entity.
+ * @param cusProduct - The customer product object
+ * @param internalEntityId - The internal entity ID to check, or undefined to check for unassigned
+ */
 export const isCusProductOnEntity = ({
 	cusProduct,
 	internalEntityId,