curl --request POST \
  --url https://api.cloud.flexprice.io/v1/subscriptions/{id}/change/preview \
  --header 'Content-Type: application/json' \
  --header 'x-api-key: <api-key>' \
  --data '{
  "billing_cadence": "RECURRING",
  "billing_cycle": "anniversary",
  "billing_period": "MONTHLY",
  "billing_period_count": 123,
  "metadata": {},
  "proration_behavior": "create_prorations",
  "target_plan_id": "<string>"
}'

{
  "change_type": "upgrade",
  "current_plan": {
    "description": "<string>",
    "id": "<string>",
    "lookup_key": "<string>",
    "name": "<string>"
  },
  "effective_date": "<string>",
  "metadata": {},
  "new_billing_cycle": {
    "billing_anchor": "<string>",
    "billing_cadence": "RECURRING",
    "billing_period": "MONTHLY",
    "billing_period_count": 123,
    "period_end": "<string>",
    "period_start": "<string>"
  },
  "next_invoice_preview": {
    "currency": "<string>",
    "due_date": "<string>",
    "line_items": [
      {
        "amount": 123,
        "description": "<string>",
        "is_proration": true,
        "period_end": "<string>",
        "period_start": "<string>",
        "quantity": 123,
        "unit_price": 123
      }
    ],
    "subtotal": 123,
    "tax_amount": 123,
    "total": 123
  },
  "proration_details": {
    "charge_amount": 123,
    "charge_description": "<string>",
    "credit_amount": 123,
    "credit_description": "<string>",
    "currency": "<string>",
    "current_period_end": "<string>",
    "current_period_start": "<string>",
    "days_remaining": 123,
    "days_used": 123,
    "net_amount": 123,
    "proration_date": "<string>"
  },
  "subscription_id": "<string>",
  "target_plan": {
    "description": "<string>",
    "id": "<string>",
    "lookup_key": "<string>",
    "name": "<string>"
  },
  "warnings": [
    "<string>"
  ]
}