Adding a stopover
 

I have a reserved ticket on the Silver Meteor leaving Philadelphia at 8:41 PM and going to South Carolina through the night. It turns out, however, that I may not need to stay in Philly all day as expected, and I would like to take an earlier regional train to Washington to spend the afternoon there, catching the Silver Meteor out of Washington later on at night (11 PM). Can I do this when I am in Philly through a ticket agent, and would I have to pay a big fee to change my ticket? I booked the trip using the 10% off sale on the internet.

Thanks for your help.