I just did this today. First I booked online the new outbound flight as a one way ticket. I then called Frontier at 801-401-9000 to tell them I would not be on the 1st leg of the round trip ticket. He said normally there would be a $99 cancellation fee, but since the tickets were less than that and I called to let them know we would not be on that first plane, he would waive the fee. I did not mention to the guy on the phone that I had booked a different flight. Just said we wanted to let them know we wouldn't be on that flight, but still needed the return leg.