Asked this in the newbie thread but didn't get a response.

I've got a refundable main ticket booked for a few weeks time from SFO - ATL (via MSP). My aim is to get an earlier direct flight on a different airline, however timing is tight and I might miss that.

At what point can I cancel the SFO - ATL ticket? Or rather, what is the latest point I can cancel it? Also, if I forget to cancel it and manage to get on the other flight, will it auotmatically be cancelled and refunded if I no-show? Does it make a difference if I check-in on the app?

I don't usually like doing this, but the earlier direct flight gets me in 3hrs before this one and I'm quite short on time in ATL.