diff --git a/src/resources/Payment/Payment.ts b/src/resources/Payment/Payment.ts
index ebd7f97..ad189cf 100644
--- a/src/resources/Payment/Payment.ts
+++ b/src/resources/Payment/Payment.ts
@@ -43,7 +43,7 @@ export default class Payment {
 
   public ChangePlan(params: IChangePlanRequestParams) {
     return {
-      get: () => {
+      GET: () => {
         return this.client.get<IChangePlanGetResponse>(
           `/payment/change_plan/${params.plan_path}`,
           {
@@ -55,7 +55,7 @@ export default class Payment {
         )
       },
 
-      post: () => {
+      POST: () => {
         return this.client.post<IChangePlanPostResponse>(
           `/payment/change_plan/${params.plan_path}`,
           {
diff --git a/src/resources/Payment/types.ts b/src/resources/Payment/types.ts
index 6905256..ac2d59d 100644
--- a/src/resources/Payment/types.ts
+++ b/src/resources/Payment/types.ts
@@ -56,7 +56,7 @@ export interface IVoucherInfoResponse {
     code: PlanCode
     group_code: PlanGroupCode
     hd_avail: PlanDiskSize
-    simulated_expiration: PlanExpirationDate
+    simulated_expiration?: PlanExpirationDate
   }
   new_remaining_days: number
 }
@@ -112,6 +112,17 @@ export type PaymentProviderName = PaymentProvider['provider']
 
 export type PaymentType = PaymentProvider['type']
 
+export type PaymentOption = {
+  name: PaymentType
+  suitable_plan_types: (PlanType | 'trial')[]
+  default?: boolean
+  discount_percent: number
+}
+
+export interface IPaymentOptionsResponse {
+  options: PaymentOption[]
+}
+
 export interface IChangePlanRequestParams {
   plan_path: string
   payment_type: PaymentType
@@ -119,19 +130,66 @@ export interface IChangePlanRequestParams {
   confirmation_code?: string
 }
 
-export interface IChangePlanGetResponse {}
+export interface IChangePlanGetResponse {
+  current_plan: {
+    plan_type?: PlanType
+    subscription_payment_provider?: PaymentProviderName
+  }
+  target_plan: {
+    price: number
+    plan_type: PlanType
+    period_days: number
+    plan_code: PlanCode
+    plan_name: PlanName
+    hd_avail: PlanDiskSize
+    simulated_expiration?: PlanExpirationDate
+    new_code: PlanCode
+    is_trial_subscription: boolean
+    subscription_trial_period?: number
+  }
+  Paddle: {
+    charge_amount: number
+    currency: string
+    next_billing_date: string
+  }
+  Fastspring: {
+    prorated_amount: number
+    refund_amount: number
+    charge_amount: number
+    currency: string
+  }
+  new_remaining_days: number
+  prorated: number
+  credit: number
+  charge_amount: boolean
+  currency: string
+  is_product_change: boolean
+  discount?: {
+    discount: number
+    type: 'percentage' | 'amount'
+  }
+}
 
-export interface IChangePlanPostResponse {
+type ChangePlanURLsResponse = {
   urls: PaymentProvider[]
 }
 
-export type PaymentOption = {
-  name: PaymentType
-  suitable_plan_types: (PlanType | 'trial')[]
-  default?: boolean
-  discount_percent: number
+type ChangePlanConfirmationResponse = {
+  confirmation: true
 }
 
-export interface IPaymentOptionsResponse {
-  options: PaymentOption[]
+type ChangePlanSubscriptionUpgradeDowngradeResponse = {
+  charged_amount: string
+  next_payment: {
+    amount: string
+    billing_date: string
+  }
 }
+
+type ChangePlanEmptyResponse = {}
+
+export type IChangePlanPostResponse =
+  | ChangePlanURLsResponse
+  | ChangePlanConfirmationResponse
+  | ChangePlanSubscriptionUpgradeDowngradeResponse
+  | ChangePlanEmptyResponse