curl --request PUT \
  --url https://api.mnemom.ai/v1/admin/users/{user_id}/billing/plan \
  --header 'Authorization: Bearer <token>' \
  --header 'Content-Type: application/json' \
  --data '
{
  "plan_id": "<string>",
  "reason": "<string>"
}
'

{
  "success": true,
  "plan_id": "<string>"
}