IMO, the most important reason for a top entrance in the winter isn’t really for an entrance, it’s for ventilation of moisture. The bees will respire and generate moisture in the box. Like with a human bathroom, if that moisture isn’t vented, you’re going get mold and wetness. Wetness defeats insulation.
Hence I’m not sure I would go to all the bother of switching out your old brood box for a new one with a hole, you can use the traditional inner cover for a vent or if you wanted a top entrance another option would be to build a custom short box that could serve as a winter top entrance and candyboard if you plan to do any candy this winter. The problem with simply putting a hole in the brood box is the hole really isn’t big enough for a colony of bees in the fall; too small. The bee population by fall is huge. It might be ok later in the winter after the summer bees have died off.
Personally I like an adjustable top entrance slot myself. I can adjust the slot wide for October and narrow it down when it gets really cold in December. That way the bees don’t overheat during an “Indian summer” warm up, and don’t freeze from a sudden cold spell (if I adjust the slots). The slot gives you more flexibility, but honestly the bees can take a lot of abuse! Just about anything will work as long as you have some kind of top venting.