> ## Documentation Index > Fetch the complete documentation index at: https://docs.fynn.eu/llms.txt > Use this file to discover all available pages before exploring further. # Preview a tier switch > Return a financial preview for switching the subscription into a target tier, with optional quantity overrides. This call does not change anything. ## OpenAPI ````yaml post /api/customer/subscriptions/{id}/plan-tier-preview openapi: 3.1.0 info: title: Fynn V2 Plans API version: 2026-06 description: >- REST API for the Fynn V2 Plans. Use it to define plans with upgrade and downgrade tiers, sell them through checkout links, and let customers switch tiers from your customer portal. All amounts use the minor or decimal representation documented per field. Authenticate setup endpoints with an API token and customer endpoints with a customer token. servers: - url: https://coreapi.io description: Production - url: https://preview.coreapi.io description: Sandbox security: - ApiToken: [] tags: - name: Plans description: Create and manage plans, tiers and items. - name: Checkout Links description: Sell a plan tier through a hosted checkout link. - name: Plan switching (customer) description: Customer facing upgrade and downgrade between tiers. paths: /api/customer/subscriptions/{id}/plan-tier-preview: post: tags: - Plan switching (customer) summary: Preview a tier switch description: >- Return a financial preview for switching the subscription into a target tier, with optional quantity overrides. This call does not change anything. operationId: previewPlanTierSwitch parameters: - name: id in: path required: true schema: type: string format: uuid example: 1280792f-4e23-793d-8e9c-3a1bf36630bd description: The subscription UUID. requestBody: required: true content: application/json: schema: $ref: '#/components/schemas/PreviewPlanTierSwitchInput' example: planTierId: f5ece259-08ea-77a4-8bd9-9f3a9ed9e246 items: [] responses: '200': description: The switch preview. May be null when no schedule exists yet. content: application/json: schema: $ref: '#/components/schemas/SwitchPreview' '401': description: Authentication is missing or the token is invalid. content: application/problem+json: schema: $ref: '#/components/schemas/ProblemError' '403': description: The token is valid but lacks the required permission. content: application/problem+json: schema: $ref: '#/components/schemas/ProblemError' '404': description: The requested resource does not exist for this Organisation. content: application/problem+json: schema: $ref: '#/components/schemas/ProblemError' '422': description: The request failed validation or a business rule was violated. content: application/problem+json: schema: oneOf: - $ref: '#/components/schemas/ValidationError' - $ref: '#/components/schemas/ProblemError' security: - CustomerToken: [] components: schemas: PreviewPlanTierSwitchInput: type: object required: - planTierId properties: planTierId: type: string description: UUID of the target tier to preview. items: type: array items: $ref: '#/components/schemas/PlanTierItemQuantityInput' default: [] description: Optional quantity overrides for editable items of the target tier. SwitchPreview: type: object description: >- Financial preview of a tier switch. creditAmount and chargeAmount are absolute decimal strings, the direction is implied by the field name. properties: creditAmount: type: string example: '14.50' description: >- Credit for unused time on the current items. 0.00 when the target tier does not credit. chargeAmount: type: string example: '24.50' description: Charge for the partial period on the new items. currency: type: string example: EUR effectiveAt: type: string format: date-time description: >- Date the switch is calculated for. For end_of_period tiers the preview uses the current moment, so the amounts are an approximation. recurringTotal: $ref: '#/components/schemas/RecurringTotals' warnings: type: array items: type: string example: - active_discount_will_be_lost description: >- Hints about side effects, for example that an active discount will not carry over. ProblemError: type: object description: >- Problem detail error body (application/problem+json). Additional members may be present depending on the error. properties: status: type: integer example: 404 type: type: string description: Stable machine readable error key. example: PLAN__NOT_FOUND title: type: string example: An error occurred detail: type: string example: Plan not found. required: - status - type - title - detail ValidationError: type: object description: >- Validation error body (application/problem+json) returned when input fails field level validation. properties: status: type: integer example: 422 type: type: string example: https://tools.ietf.org/html/rfc2616#section-10 title: type: string example: Validation Failed violations: type: array items: type: object properties: propertyPath: type: string example: tiers[0].name message: type: string example: This value should not be blank. code: type: string nullable: true required: - status - type - title - violations PlanTierItemQuantityInput: type: object required: - planTierItemId - quantity properties: planTierItemId: type: string description: UUID of the tier item to override. quantity: type: number minimum: 0 description: >- Requested quantity. Must lie within the item minQuantity and maxQuantity and the item must be quantity editable. RecurringTotals: type: object description: Recurring amount for one billing period. Amounts are decimal strings. properties: netAmount: type: string example: '49.00' taxAmount: type: string example: '9.31' grossAmount: type: string example: '58.31' currency: type: string example: EUR securitySchemes: ApiToken: type: http scheme: bearer description: >- Organisation API token. Send it as Authorization: Bearer api_... The token is scoped to a single Organisation. CustomerToken: type: http scheme: bearer description: >- Customer access token. Send it as Authorization: Bearer . Obtain it server side via POST /customers/{id}/authenticate (use the response accessToken), or from a customer portal session. The token must carry the customer:plan:switch permission. ````