Depends on how important flexibility is to you, and what kind of changes you might want to make. On the one hand, if you cancel the return of a RT, the credit is nearly impossible to use. On the other hand, if you just seek to change date for the return of a RT after flying the outbound, you typically get the benefit of the original fare pricing.
That will require cancelling the two one-ways and using the flight credits to book a brand new ticket. As I see jsloan has explained, you can probably do all that online.