Hi
I would say if your going to get the fascia sorted i would get the old ones off and render right up so that once it is feathered it will be flat for your new ones.
With it going into a bell at the bottom yes this can happen but it is how you apply it to the surface if done correctly this should not happen.
You can also have different types of fascia, sometimes if you have timber ones there and they are set in and not rotten etc might be worth leaving them in and going on top of them with pvc jobbies.
Good luck with what you do if you need any more info/help let me know.
Cheers
Paul