The ceramic coating adds a tiny bit of thickness to the case. For stabilizers that were already tight without the coating, it can be the difference between them fitting or not. I have had success very carefully filing/sanding off the coating in the stabilizer slots to make tighter ones fit. Just be careful to not remove too much material if you attempt this.
You only need to buy a new diffuser if your keyboard is Massdrop branded (on the bottom). Otherwise yes, just take it apart and swap the components over to the new case.