mutation PreviewBillingPlanChange($input: PreviewBillingPlanChangeInput!) {
  previewBillingPlanChange(input: $input) {
    preview {
      immediateCost {
        amount
        currency
      }
      earliestEffectiveAt {
        unixTimestamp
        iso8601
      }
    }
    error {
      message
      type
      code
      fields {
        field
        message
        type
      }
    }
  }
}

{
  "input": {
    "planKey": "LEGACY"
  }
}

{
  "data": {
    "previewBillingPlanChange": {
      "preview": {
        "immediateCost": {
          "amount": 1,
          "currency": "USD"
        },
        "earliestEffectiveAt": {
          "unixTimestamp": "string",
          "iso8601": "string"
        }
      },
      "error": {
        "message": "string",
        "type": "VALIDATION",
        "code": "string",
        "fields": [
          {
            "field": "string",
            "message": "string",
            "type": "VALIDATION"
          }
        ]
      }
    }
  }
}