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

{
  "success": true,
  "subscription_id": "<string>",
  "old_plan": "<string>",
  "new_plan": "<string>",
  "credits_adjusted": 123,
  "message": "<string>"
}