If I want to change my license from one class to another (in the near future), would I need a Pre-Licensing Course certificate?
No. You don't need a PLCC, as long as you have a valid NY state driver license you can apply to change your class.
One more question, you said before that accelerating smoothly and moderately was key, but what if I have to drive on a highway in my test? I can't go moderately, since all the vehicles will be honking since I'm going slow.
Don't worry about that! When you are taking the driving test, your headlights will be on so everyone know you are in a test, also the car we use is a special car and has a logo. So it's ok, just stick to a smoothly and moderately acceleration, and don't force the engine so it does not stall.