curl --request POST \
  --url https://api.example.com/speakers/undo-reassign \
  --header 'Content-Type: application/json' \
  --data '
{
  "old_assignments": [
    {
      "transcription_id": 123,
      "old_speaker_id": 123
    }
  ]
}
'

{
  "restored": 123
}