For a trip that hasn't started, you can cancel and redeposit the entire award and then rebook the one-way flight as a standalone at current mileage levels. It's the same thing that would happen if you had booked a roundtrip revenue fare. Note that there are award levels that require roundtrip bookings (with higher levels required for one-way award bookings). Letting one keep a one-way award after booking a roundtrip in such cases would defeat the roundtrip booking requirement for these award levels. Although this doesn't apply in your case, once travel on a roundtrip award booking has begun, you can't redeposit the return award (even if they are one-way award levels).