curl --request POST \
  --url https://api-eu.flexportal.io/v1/subscriptions/{subscriptionId}/upgrade \
  --header 'Authorization: Bearer <token>' \
  --header 'Content-Type: application/json' \
  --header 'Tenant-ID: <tenant-id>' \
  --data '
{
  "tenantId": "<string>",
  "rentalId": "<string>",
  "newSku": "<string>",
  "newSerialNumber": "<string>",
  "contractLength": 60,
  "newMonthlyAmount": 1,
  "reason": "<string>",
  "notes": "<string>"
}
'

{
  "success": true,
  "message": "<string>",
  "oldRentalId": "<string>",
  "newRentalId": "<string>",
  "oldDevice": "<string>",
  "newDevice": "<string>",
  "newMonthlyAmount": 1,
  "currency": "<string>"
}