Just call AC and cancel the first leg. For a basic fare you forfeit the fare. For Standard fares and above you get a credit. For domestic fares there are no return requirements. Ie. the return leg won’t be repriced. There are no cancellation fees. AC only charges change fees when you actually use the credit. There will be no credit for a basic fare anyway.

I did did the last year for my son. 5 minute call to AC.