it's a bit more complicated

When you have a cancelled trip, you pay a changed fee to reuse the previous credit (hence the term "change" fee). The funds for the original ticket are locked into the old reservation. After making the changes, if the original ticket was more expensive you then receive a vouchure that can be used on a future ticket.

So with two cancelled tickets, you will have to paid two change fees and need to make two new reservations. If you really only need one reservation then make the other unneeded reservation for the cheapest flight you can find (such as an OW SFO-LAX or AUS-IAH ) and get an e-voucher to fund a future trip or to cover extra cost in the second changed reservation.