gistfile1.txt

File 1:

REMOVE: LegacyRecord::each(function($record) {
ADD: LegacyRecord::with('new_record')->each(function($record) {
  $identity = ['legacy_id' => $record->id];
  $values = $this->transformLegacyRecord($record);

REMOVE:  NewRecord::updateOrCreate($identity, $values);
ADD:  $new_record = $record->new_record ?? NewRecord::make($identity);
ADD:  $new_record->fill($this->transformLegacyRecord($record));
ADD:  $new_record->save();
});

File 2:

class LegacyRecord extends Model
{
ADD:    public function new_record()
ADD:    {
ADD:        return $this->hasOne(NewRecord::class, 'legacy_id');
ADD:    }
}