curl --request POST \
  --url https://api.example.com/v1/billing/preview-change-plan \
  --header 'Authorization: Bearer <token>' \
  --header 'Content-Type: application/json' \
  --data '
{
  "new_tier": "<string>",
  "new_interval": "<string>"
}
'

{
  "current_plan": "<string>",
  "new_plan": "<string>",
  "proration_amount": 123,
  "credit_adjustment": 123,
  "effective_date": "2023-11-07T05:31:56Z",
  "message": "<string>"
}