The pricing can be very different, and I believe when you change up your tickets you need to pay the current price on Amtrak. My daughter took a train back to college purchased in advance, $30, her friend got a last minute ticket for $100. Two of my kids used NJ transit trains to travel back and forth to college, so easy (one accidentally booked Amtrak the first time, fortunately he also booked the insurance so he was able to cancel 🤣).