How does the My Trip function on the Amtrak site work?

So I am planning a rail pass trip and was starting to add my routes and realized I screwed up and got the dates wrong. There is a cancel button on each ticket and gave me a scary warning. I called the Amtrak support line and the agent was able to delete them.

I guess my question is, can I delete/add these up until a certain date? I know many sell out so I'm obviously not going to cut it close by any means but are you able to delete them from the web site without calling in?