I'm A Banana
Potassium-Rich
In order to make Harry Potter d20, you'd either have to do disservice to the setting, cutting and rearranging so that it can squeeze in where Redhurst fits naturally, or you'd have to do a lot of cutting, rearranging and addition of the d20 rules themselves.
You don't make Harry Potter D&D, you make Harry Potter d20. The two are quite different. Rules-light is all about how detailed you make the combat.

Redhurst made Harry Potter D&D. Harry Potter d20 would be different. It would have a different magic system. It would probably alter the combat to emphasize magic. It would have a balance with the assumption of magic for every creature (without being a Wizard or Sorcerer or other d4 blasting paper tiger). It would be a different game than D&D, it would play different, feel different, and have different challenges. It wouldn't be kids fighting orcs in merry olde England.
To truly do justice to HP in RPG form, you need to capture the sense of myth and legend about the world and reality of HP. You need to have darkness and mystery just beyond the walls. You need to play with the idea of having magic but not getting everything you want with the wave of a wand. Play with issues of maturity, growing, pain, loss, and ability. D&D is a sublimely designed system for killing things and taking their stuff. HPd20 would be a game designed for facing the dangerous world behind the everyday. And there's no reason d20 would be any worse at doing this than any other system, straight out the box.