Sandy,
Question for you. How much time did your arrival time into Philly change? If it's at least 30 minutes or more, you most likely can fight with the airline to change your flight for free. However, you need to stay firm with this. YOU did not change the arrival time into Philly; they did. If you wanted to arrive at a later time, then you would have selected a later arrival time. If the airline agent gives you a hard time (if it's US Airways, they may), then ask to talk to a Supervisor. Don't get off the phone until they change it for you for free. I've done this many times and its worked.
Hope this helps and keep us posted!
Lorrayne