ALTER TABLE training_cancellations
  ADD COLUMN IF NOT EXISTS start_date DATE NULL,
  ADD COLUMN IF NOT EXISTS end_date DATE NULL;

UPDATE training_cancellations
SET
  start_date = COALESCE(start_date, date),
  end_date = COALESCE(end_date, date)
WHERE start_date IS NULL OR end_date IS NULL;

ALTER TABLE training_cancellations
  MODIFY start_date DATE NOT NULL,
  MODIFY end_date DATE NOT NULL;
