score:1
What you are trying to do is "hidden city ticketing" and it's forbidden by AA's contract of carriage, so I would not expect them to make it easy for you to accomplish this.
I would think that if you try to split the itinerary, they will compute the total fare for one-way trips EWR-PHL and PHL-PIT and then charge you for the difference above what you have already paid, perhaps in addition to a change fee of $100-$200 if your original ticket was non-refundable. So this is even worse than just changing your itinerary to PHL-PIT.