I would be more optimistic than the posts above. I would expect that you have a better than 50% chance of getting a ticket agent, at the airport, to give you seats on those flights IF there are still open seats on both legs when you show up in the morning and IF you ask nicely. I've changed my connecting city at the starting airport before.

As mentioned above, try and show up at the airport EARLY to try and get this done.

Please let us know how it turns out.