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.
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.