take the $downID->sort_id to a separate variable

Or you could use a trait

<?php

namespace App\Common\Models;
use DB;

trait PositionUpdate {

    public static function bootPositionUpdate () {
        static::created(function($item){
            if (!isset($item->positionUpdateFilter)){
                throw new \Exception('$item->positionUpdateFilter must be set');
            }

            if (!is_numeric($item->position)){
                $item->position = DB::table($item->table)->max('position') + 1;
            }

            $sql = "UPDATE $item->table
            SET position = position + 1
            WHERE position >= $item->position
            AND id != $item->id
            AND $item->positionUpdateFilter = " . $item->{$item->positionUpdateFilter};
            $update = DB::select($sql);
        });

        static::created(function($item){
            if (!isset($item->positionUpdateFilter)){
                throw new \Exception('$item->positionUpdateFilter must be set');
            }
            $sql = "UPDATE $item->table
            SET position = position + 1
            WHERE position >= $item->position
            AND id != $item->id
            AND $item->positionUpdateFilter = " . $item->{$item->positionUpdateFilter};
            $update = DB::select($sql);
        });

        static::updated(function($item){
            if (!isset($item->positionUpdateFilter)){
                throw new \Exception('$item->positionUpdateFilter must be set');
            }
            if ($item->getOriginal('position') < $item->position){// move up
                $sql = "
                UPDATE $item->table
                SET position = position - 1
                WHERE id != $item->id
                AND position > " . $item->getOriginal('position')."
                AND position <= $item->position
                AND $item->positionUpdateFilter = " . $item->{$item->positionUpdateFilter};
                $update = DB::select($sql);
            } else {// move down
                $sql = "UPDATE $item->table
                SET position = position + 1
                WHERE id != $item->id
                AND position >= $item->position
                AND position < " . $item->getOriginal('position') . "
                AND $item->positionUpdateFilter = " . $item->{$item->positionUpdateFilter};
                $update = DB::select($sql);
            }
        });
    }
}

add trait and $positionUpdateFilter to model and ensure the db table has a 'position' integer field

personally i don't recommend using traits, they are so ugly. sorry ,. my personal preference