You cannot be checked in to do SDFC. In the app choose modify check-in and then I think the option is to "cancel checkin" or "uncheckin".

Once you are no longer checked in, choose manage trip and then change. Search for flights on the same day. It should offer the ability to change for $0.

I suspect using the chat was a mistake. Unless there is chat specific for Mosaic customers, they might not understand the Mosaic benefits. If I need a human, it's the Mosaic phone support number for me.